Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net



Tuesday, 15 July 2014

Review : God's Philosophers


Science in the medieval period was virtually non-existent. The Catholic Church did not take at all kindly to anything that contradicted its rigid, inflexible teachings, and the few people brave enough to speak out were summarily dismissed as heretics and burned at the stake. At least, that's what we're all taught. And that's what many people today would like to believe. But it simply isn't true, says Dr James Hannam, graduate of both Oxford and Cambridge, in his fascinating 2009 book God's Philosophers. You can read the first two chapters of the book for free on his website.

Rather than a detailed review of the style of the book, instead I'll give a short summary of some of the aspects I found more interesting. I'm going to concentrate on the philosophical and moral aspects of the relationship between science and religion; if you want more direct examples of how highly devout medieval Christians contributed to modern science, you should read the book itself.

As far as style goes, suffice to say it's very accessible, with a good balance between science, philosophy and theology. You don't need to be a scientist, philosopher or theologian to understand it. However, to get anything out of the book you'll have to be prepared to surrender many of the impressions of the medieval Church you've probably grown up with. If you're one of those people convinced that only science has led us out of the darkness of religion and into the light of reason, well now's a perfect time to put your money where your mouth is. You're not allowed to extol the virtues of science unless you're prepared to question your own viewpoints - otherwise you ain't no scientist at all, boyo.

Hannam is under no illusions that science (or natural philosophy as it was then) and religion did not sometimes come into direct conflict, and that when push came to shove, the Church had the upper hand. But the extent to which that meant the Church actively suppressed freedom of thought has been vastly exaggerated, and the achievements of medieval theologians (yes, theologians) that were crucial for the breakthroughs of Copernicus, Kepler and Galileo - who were all devout Christians - have been unfairly airbrushed from history. It's not an attempt to convert anyone, only to point out the debt modern science owes to medieval Christian thinkers. They not only rediscovered the works of ancient philosophers, but surpassed them.


Theology, Doubt, and Natural Philosophy

As a fervent and unswerving agnostic, the concept I found most interesting from the book was the idea that theology and science were closely linked. Although I was aware that medieval universities existed, I've normally dismissed them as being irrelevant theological schools, producing nothing of any real consequence. This was a huge mistake. Theological training, says Hannam, not only included the study of mathematics and natural philosophy, but was virtually obsessed with logic. The underlying idea, it seems, was that by studying the natural world one could understand God. Natural philosophy was seen as, "the handmaiden of theology" - not perhaps a very flattering label, but completely at odds with the (mis)conception that science and the Church were mutually exclusive.

The obsession with logic produced some genuinely very interesting consequences. Could God create a weight so heavy he couldn't lift it ? No, said the theologians - that would be a logical contradiction, and even God isn't that omnipotent. Moreover, not even the most devout scholar (and remember that these people were members of the Church) believed in the literal truth of the entire Bible. They couldn't, because that is simply impossible. If you took the Biblical phrase, "four corners of the Earth" literally, you'd believe it was flat*. The idea of interpreting the scriptures figuratively goes at least as far back as St (emphasis : Saint) Augustine, 354 - 430 AD.

* Which they didn't, even by 1000 AD - this is a more recent myth about medieval scholarship. Other examples included various passages which state that the Earth does not move, the interpretation being here that it meant "from the perspective of someone standing on the Earth".

In a somewhat roundabout and almost perverse way, the Church actually encouraged freedom of thought. While initially the teachings of Aristotle were almost treated as Gospel, the omnipotence of God would allow him to create anything he liked (so long as it wasn't a logical contradiction). So it was perfectly fine to contemplate vacuums (which Aristotle didn't believe in) since, even though they might not exist naturally, God could potentially create one if he wanted. By today's ideologies this is a truly bizarre way of overcoming difficulties with mainstream theories, but it was certainly better than assuming the ancients had already got natural philosophy licked - most of Aristotle's ideas were complete gibberish. His prestige kept his ramblings at the forefront of scientific theory long after they should have been rejected, but medieval inquiry was (albeit very, very slowly) able to come up with better ideas.

Interestingly, God's ability to do as he pleased in no way hindered investigations into the workings of the natural world. God was seen as the primary cause of all things, but he usually operated by invoking secondary, natural causes that proceeded to operate with strict rules. Moreover, if God didn't like what you were up to, he was free to cause a miracle to stop it. If you were sick, you could try using magical remedies to get better, but if God wanted you to be sick, then sick you would damn well stay*.

* At least one popular image of the medieval world appears to be entirely correct : doctors were something to be avoided like the... err, plague. Prayer actually did have a much better chance of success - supernatural deities aside, at least priests weren't going to bleed you half to death as a "cure".


The Burden of Proof

When something was found that disproved a statement in the Bible, or a decree by the Church, this was accepted. The flat Earth is one example, the fact that the antipodes are inhabited (a notion condemned by the Pope sometime in the 8th century) is another*. When the level of proof was 100% certain - as in finding people living in the antipodes - then even matters of faith had to give way to rational science.

* The Church was, however, quite right that the hot, sweaty tropics are uninhabitable. The fact that people stubbornly continue to live there anyway is beside the point.

A running theme of the book is that (for instance) the image of the Earth being at the center of the Universe was the most rational, logical viewpoint given the evidence available at the time*. Proving the Earth is round is easy; proving it goes round the Sun with only the evidence of your eyes and nothing else is very, very hard. It's extremely difficult to see the world as medieval thinkers would have, though Hannam makes a valiant effort. Of course, we know today that the acceleration of a rotating Earth isn't sufficient that we can all feel ourselves whirling round at tremendous speed, but at the time, that was a logical, sensible reason to reject the notion of a rotating Earth.

* Interestingly, Earth wasn't placed at the center to reflect its importance - quite the opposite. Heaven was the most important place (the medieval Universe believed to be about 90 million miles across), so anything further away from the surface of the Earth was closer to God. Conversely, anything deep in the ground was closer to Hell and therefore worse. We were literally living on the surface of a "Middle Earth", if you like.

Hannam makes the important point that science cannot function where every idea requires absolute, irrefutable proof. For instance, I wasn't around when the Earth started forming, so I cannot proove God didn't do it. But to make any further progress, I must assume (based on other lines of inquiry and well-tested theories) that this is the case, and proceed from there. That is the essence of a scientific principle, something which unfortunately seems to have largely escaped the medieval mind. Having speculative models was fine, as we'll see, but being allowed to assume they were true was quite another. When it came to theorising, religion definitely had the upper hand over natural philosophy. And that caused some very acute unpleasantness when Copernicus's model of the Solar System was found to be much more accurate than the then-mainstream Earth-centered view.


The Limits of Freedom

There were most certainly limits on what ideas the Church would put up with - cross them, and you would indeed meet with a very nasty fate indeed. But those limits were very much larger than I realised.

The Inquisition wasn't a good thing, but don't confuse the Papal and Spanish Inquisitions. The latter (see Toby Green's Inquisition) was a brutal, oppressive system that was largely politically motivated, and did indeed place very tight restrictions on what you could and could not say. The former, however (as Green also says) was quite different. If you admitted your "crime" (and by today's standards of course they would not be crimes at all) to a Papal inquisitor, you'd be set free (though woe betide repeat offenders). Not so with the Spanish Inquisition, where torture was common and confession still meant death more often than not. The Inquisition of the Papacy was not a nice thing, but the Spanish Inquisition was far, far worse.

So what would it take to be declared a heretic ? Well, quite a lot, actually. You could speculate about almost anything, so long as you were clear to state, "this is just an idea, I don't know if it's really true." Take Cardinal Nicholas of Cusa (1401 - 1464), who had the notion that maybe the Universe was limitless and the Earth was just another star moving through space, and not in the center at all. He even postulated the existence of aliens. He was a Cardinal, for crying out loud, and no-one thought anything amiss with this. It's worth noting that this idea was based on the idea that the Universe would have to be limitless to "reflect God's majesty", not for any scientific reasoning. There was absolutely no scientific observation (or even discussion - Olber's Paradox being centuries in the future) at the time that gave any evidence of an infinite Universe.

What about Giodarno Bruno (1548 - 1600), that noted visionary burned at the stake for believing that the Earth wasn't at the center of the Universe ? Nope. Bruno was, quite simply, a nutter.  He seems to have tried to come up with an entirely new religion based on magic, plagiarised more competent thinkers (though Galileo also did this) whose mathematics he simply couldn't understand, and had the annoying habit of loudly telling everyone he was a genius. Basically, he would be this guy :


Now, being a bit of a jerk and believing in magic are pretty stupid reasons to convict someone, but, although he did believe the Earth went around the Sun, that's not what he was investigated for - it wasn't declared a heresy until 16 years after his death. Unfortunately, the list of charges has not survived, so we can't be sure what the real charges were. Certainly, Bruno was an example of the Church behaving at its worst and this is clearly an example of suppression of freedom of thought, but Bruno wasn't a martyr for science by any stretch of the imagination. He was a magician and a mystic, with no more claims to scientific genius than Cardinal Nicholas.

Not that even heresy was always a guaranteed immolation, mind you. Virgil of Salzburg (700 - 784) not only got away scot-free for teaching the antipodes were inhabited by people not descended from Adam, but was later even canonized. William of Ockham (he of the razor) also escaped, although in this case quite literally by hiding under the protection of the Holy Roman Emperor. And Galileo (more on him in a minute) was, for a while, allowed considerable leniency by a somewhat corrupt and capricious pope.

Before returning to Galileo, it's worth noting one very clear example of religious doctrine impeding scientific inquiry. Nicholas of Autrecourt (1300-1369 AD) attempted to claim that everything was made of atoms. Unfortunately, this appeared to make the transubstantiation in the Eucharist (bread into flesh and wine into blood) not merely miraculous, but a logical contradiction - something cannot appear to be made of bread but really made of atoms of flesh. Nicholas was forced to recant and the offending document burned, but he himself gained a cushy job as a dean and lived more or less happily ever after. Hannam contends that this is a rare example, and if he'd only not made the link to the Eucharist so explicit and just been a little more careful to emphasise that it was "only an idea" (which would have been sensible given the lack of definitive evidence at the time), everything would have been tickety-boo. He doesn't use the words "tickety-boo" though, which is a shame.


And Yet It Moves ?


Galileo is the archetypal heretical scientist. Unlike Bruno, he was certainly no mystic, and his ideas were based on solid observational evidence. Ultimately, this one does boil down to science versus religious ideology... well, sort of. Maybe. It's far from as open-and-shut as you might think.

Many discoveries essential for Galileo's work had already been made. Copernicus had published his Sun-centered Solar System model (dedicated to the Pope) in 1543, with a careful foreword by his friend Osiander stating that it was only a hypothesis. Interestingly, this seems to have been more to ward off scholarly skepticism than Church wrath. At the time, the model seemed at odds with the observation that the constellations do not change throughout the year. Since the medieval Universe was only 90 million miles across, if the Earth moved through a significant fraction of that, the constellations would appear distorted as it approached the sphere of the fixed stars.

Copernicus's solution was the correct one, but so dramatic it was very hard for everyone else to accept. To reconcile the theory and observations, he increased the distance to the stars by a factor of a billion. Changing theory has never been a popular move - the idea you need to create multiple Universes to kill a cat still causes problems for quantum theorists today, as does the idea of changing Newtonian gravity to fit observations of galaxies. We seem to have an innate tendency to prefer to think that we're basically on the right lines, unless the evidence becomes overwhelming. And in 1543, Copernicus' case was not overwhelming.

But it worked. There seems to have been little or no ecclesiastical backlash (perhaps because the book was so mathematical, suggests Hannam) and by the 1570's a Papal commission was using the Copernican model to develop our modern calendar. There was no getting around the fact that it produced much better results than the alternatives, though it seems that people were happy to accept it as "only a theory". The trouble only began when people began to believe it might also be a true description of reality.

By 1588, Tycho Brahe had demonstrated that the planets were not moving in giant crystal spheres. His model of the Solar System still had the Earth at the center, but the planets orbited the Sun rather than the Earth. In 1596, his protégé Johannes Kepler published a model with the Sun at the center, and no-one seemed to mind. Very few people believed the idea, but some priests stated that it while it was religious unobjectionable, it was scientifically unsound.

So three great scientists had already postulated alternative models of the Solar System that directly contradicted scripture and no-one had even got so much as lightly singed. For a long time, Galileo also got along just fine, conducting scientific observations and publishing them in what amount to 17th-century popular science books. He was also on good terms with the Pope. As we've seen, no-one at the time was particularly bothered by Copernicus or Kepler's theories.

That all changed though with a particularly strict Inquisitor, Cardinal Bellarmine. Galileo held the opinion (as did many others, theologians included) that the Bible should be taken figuratively except in matters of morals - leading to the famous quote, "The intention of the Holy Spirit is to teach us how one goes to heaven, not how the heavens go.". The official Council of Trent had said much the same thing.

Bellarmine was having none of it. His view was, unusually, that the Bible was the literal word of God and should be taken as such unless absolutely irrefutable proof was given. The Italian friar Foscarini had recently forced the issue by writing a letter tackling the problems with scripture and the movement of the Earth head-on. It failed utterly. His letter was banned and Copernicus' work suspended pending corrections. Galileo got off with a warning not to teach Copernicus' theory. Hannam's implication (he does not say this explicitly) is that Bellarmine was the main proponent responsible for turning the previously unobjectionable Copernican model into a heresy, but I would have liked a lot more detail on this point.

Four years later, in 1620, the Church released the corrected version of Copernicus' work. It was hardly a ruthless censorship (as the Spanish Inquisition would have done) - ten corrections, released as a special insert, in a book hundreds of pages long. Hannam gives the example that "admitting the Earth moves" became "assuming the Earth moves", emphasising that it was just an idea, not a proven fact. Which, after all, it wasn't - this is scarcely worse than having a paper or thesis go through peer review today (albeit with potentially more extreme consequences in the case of failure to comply).

Bellarmine died a year later, and soon after Galileo began work on a a popularisation of the Sun-centered Solar System, confident that as long as he was careful, his friend the Pope would support him. And perhaps he would have done, had he not (according to Hannam) made a single catastrophic error of judgement, by what amounted to mocking the Pope. It's worth noting, however, that initially it did pass muster from the Papal censors and it was allowed to be published. Only when the Pope read it did things get ugly.

The problem, it seems, was that Galileo did not have a good enough explanation of tides. His idea that they were caused by the rotation of the Earth, with the water being left behind and so sloshing around. He not only tried to use this to prove the Earth rotates (even though he'd been warned not to) but for the counter-argument in the dialogue of the book he used an argument by the Pope that God can create any circumstances he wanted (basically saying that, "no, the tides are caused by the will of God"). Worse, he put this toward the end of the book, which the Pope took as adding insult to injury.

The resulting trial didn't exactly see Galileo stand up for his ideas, and I can't say I blame him. Initially claiming he didn't support Copernicus at all, when pushed he admitted that, "well I suppose some moron might read my book that way" (not a direct quote, unfortunately). No-one but believed him, but, threatened with torture, he stood his ground and called their bluff. Satisfied by his apparent conviction, the threat was withdrawn. Though he avoided the rack or a burning, he was condemned to life imprisonment under house arrest.



Conclusion

Hannam does  an admirable job of demonstrating the debt modern science owes to medieval theologians. Within this very limited scope, the book is excellent. It would have been nice to have added a least a few related points though - in particular, it's clear that Universities offered protected intellectual havens, but almost no mention is made of how academic ideas were regarded in society. Similarly, there's no discussion on what the academics thought about more broad issues : morality, the Crusades, etc. Although this lack is understandable, given the obvious subtext of the book it would have been nice to include something.

There are certainly examples from history of the Church suppressing freedom of thought, sometimes brutally. But these are rare, and in the case of scientific inquiry, practically non-existent. The ultimate weapon, the auto de fe, potentially could have been used against troublesome scientists, but it doesn't seem to ever actually have been. The fact that the Church was ever allowed to have any say over what people could publish and could punish (or even execute them, if only in principle) for stepping over the mark was not, of course, a good thing. In practise, this was extremely rare. Medieval theologians contributed far more to modern science than the Church ever held it back. At least, Hannam does a good job convincing me of this.

Maybe it's possible to argue science would have been better off without the Church, I don't know. But at the very least, the popular idea that the Church only worked to hinder progress is now simply untenable as far as I'm concerned. It's also worth remembering that even in the modern age, espousing viewpoints too far from the mainstream may not get you burned alive, but it will certainly get you laughed at and ejected from academia. Given the intensely rational nature of medieval theology, the line between heresy against scripture and unconventional ideas is, perhaps, thinner than we might like to think.

Perhaps the most glaring omission from the book, however, is any discussion relating to what happens next. Hannam concludes with the trial of Galileo. What a future version of the book is badly in need of is an epilogue to summarise the next few centuries. What I would especially like - and this would probably need a whole other book - is some discussion on how we went from being able to say, "well, obviously the Bible isn't meant to be taken literally about everything" in mainstream medieval theology, to the current view of a disturbing number of people that the Earth is only a few thousand years old.

Far from condemning modern scientists as heretics, a medieval theologian - if properly briefed on the current evidence - would almost certainly have little good to say about modern creationists. These ideas do not take us back to a medieval world view - they're much worse than that. Even Cardinal Bellarmine would be forced to concede to modern evidence. Small wonder that Frank Herbet once quipped that in religion there is "always the unspoken commandment, thou shalt not question !". Ironically, such a view is a far better description of modern extremists than it is about the logical, questioning mind of the medieval theologian.

Wednesday, 2 July 2014

Referees Who Move Goalposts Make Lousy Peers

My latest paper, The Arecibo Galaxy Environment Survey VII : A Dense Filament With Extremely Long HI Streams, has finally been accepted for publication. This has consumed, in one way or another, about the last year or so of my research time. So I'm going to indulge myself with not one, not two, but three entire blog posts. In this first one I'll look at the peer review process, which for this particular paper was slightly less fun than being given an enema by a bear. In the next post I'll describe how we went about searching for galaxies using Blender, and in the third, thrilling conclusion, I'll say something about what it is we actually learned.


Peer review does not mean writing articles in the Guardian about what you think of Jeffrey Archer, unless you're a journalist. In science, the idea behind peer review is simple : someone writes a paper, and someone else (usually anonymous and selected by a respected academic journal) checks to make sure it makes sense. In this way, scientific literature is pretty well-defended against giant space bananas and moon landing conspiracy theorists. No-one think's it's perfect - a perfectly objective system is probably impossible - but it's the only system we've got for checking that everyone else is basically sane.

In this case it took over seventh months from first pushing the big scary, "WARNING : THIS WILL SUBMIT THE ARTICLE ! ARE YOU ABSOLUTELY, POSITIVELY, DAMN WELL BLEEDIN' SURE YOU REALLY REALLY WANT TO DO THIS ?!?!?!" button to having someone say, "This looks decent, let's publish it." Almost as long as creating a baby, though less icky, and as you can imagine I'm none too happy about that.

Although now I come to think about it, if it's a choice between one or the other, I'll take the publication, thanks.


I've never had to referee a paper, mercifully. I don't imagine it would be much fun. Several weeks trying to make sense of other people's wranglings with no time for your own research can't be much of a hoot. But I have received several referee's reports on my own work. Typically, this results in my mental processes going something like this :

First reading
"Add error bars in figure 6 ? What a JERK ! No-one else puts error bars on this plot. This guy is clearly a complete amateur."
Two hours later
"Huh, errors aren't so hard to calculate after all, maybe I should add them in instead of insulting the referee's mother at the next submission..."
The next day
"Hey ! With error bars this figure looks waaay better !"

Which is precisely why this post has been in draft form for several months, but I'm not going to even try claiming an unbiased viewpoint.

Referee's comments come in all varieties, from the extremely helpful (which are probably the majority, if I'm honest) to the extremely unhelpful (more on them later). A few are just downright strange. One I've seen every single time is, "shorten the text." This is like saying, "There are too many notes, that's all. Just cut a few and it'll be perfect.". It doesn't make any sense. Tell me which bits you think are too long, or I'm just not going to do anything*.

* Actually what I'm going to do is cut out one paragraph just so I can say I've done something. What I'd like to do is remove the final word from every sentence.

What's almost worse and just as common is that the referee will simultaneously state that the paper is too long and require major additions. Referees, please don't do this. It makes you sound like a crazy person. You have to at least suggest where the cuts should be made, or it makes no sense. I can't possibly make it any shorter if you're telling me to add stuff.

In like vein, the weirdest, most incomprehensible suggestion I've received so far was that I should add "supporting figures to the paper" (there wasn't any context to this). What, you mean like a picture of a giraffe or something ? A self-portrait in crayon ? Maybe a drawing of Atlas, he's a supporting figure, after all. Love to help, but I'm not psychic ! Of course, the assume-everyone-is-psychic factor affects us all, but not usually to that degree.



Fortunately, this particular mystery was solved in consultation with a friend, who brilliantly realised :
"It means there aren't enough characters. You need to flesh out your story with supporting figures that the referees can relate with. Maybe add a downtrodden scientific paper reviewer, who started out with the bright-eyed idea of changing the world through peer reviewed science, but instead met paper after paper of nonsense and flim-flam, and now has no joy left when a paper of some real worth comes through."

Much more annoying are criticisms of the second draft that could have been made at the first submission. Now obviously there's nothing wrong with saying, :
"Oh and I forgot, you need to correct the spelling of GALAXIE on page 2 and make the caption bigger."
No-one can spot every typo at the first reading. But if you're going to suggest :
"You should replot all your figures so that they actually show something completely different, because although I didn't bother to mention it last time, I think they're all wrong. And also I don't think you wrote your code in C, so please do that."
... then that's fine too - it's the referee's job to do this - but don't suggest this at the second draft when you could do so at the first. This is unfair and unprofessional - it's moving the goalposts, which is traditionally frowned upon. Especially if it's done by a referee. Also, it really wastes a huge amount of time - I could have made the changes months ago and have the paper ready for publication already.



Perhaps the most annoying of all is to repeat verbatim criticisms of the first draft that the author quite clearly, unmistakably and - above all - directly addressed in the second draft. As in :
Referee : "You need to say how many galaxies are in your sample."
Author : "There are 5 galaxies in this sample."
Referee  : "You need to say how many galaxies are in your sample."
That's not a real example, but some comments really are every bit as inane as that. They're even worse when they're longer comments that took correspondingly longer to address. I can only assume in these cases that the referee didn't actually read the author's response - which makes it a no-win situation. If you're not going to even read my response, why should I bother making corrections at all ?

The worst example of this was a referee who insisted that there were better data processing techniques we could use. Great ! It was obvious to anyone that the method we were using, while an accepted and well-tested approach, might not be ideal. So in my response I asked the referee to provide a reference so that I could try this new approach for myself. Unfortunately in the second report the reviewer didn't answer my question at all, but instead did a copy+paste job of their previous comment.

Sigh.

But we all have bad days. In my response to the second report, I again asked (twice this time, just to make sure) for a reference to these shiny new methods. In the third report, the referee flat-out ignored this altogether, merely stating simply that our measurements must be wrong - and worse, that not only were the numbers wrong, but all of the major results were also wrong. Even though they hadn't raised any previous objections, and simultaneously stated that the paper was now "much improved".



In this case this rather brusque dismissal, at the third iteration of the paper, was particularly galling since I'd spent a very, very long time making sure those measurements were as good as I could damn well make them. The end result was quite nice, agreed with previous measurements (where available) and wasn't particularly controversial. All lines of inquiry pointed to the same result. To have the referee then suddenly dismiss them - after not raising any previous objections - without giving a clear reason why was simply too much, so we asked for a second referee.

Now, as an aside on the much-vaunted objectivity of science, it may interest readers that referee 2.0 had no qualms whatever with the data analysis, which was vindicating. Whereas referee 1.0 was perfectly happy with the introduction, referee 2.0 thought parts of it were incomplete and misleading, and provided a list of some genuinely interesting papers in support of this. Which is great, and I happily made the requested changes (although I'm not sure I agree with the referee's sentiments, a happy compromise was easily reached)... but, it does, of course, expose a basic flawed truth of peer review : not all scientists agree with each other. But referee 2.0 provided justification for their assertions, whereas referee 1.0 didn't.


Make no mistake : I support the peer-review process. And philosophically, I don't like the notion that if you disagree with the reviewer, you should find another reviewer - but let's not go nuts. Sometimes, just like everyone else, reviewers are just jerks. Peer review may not be the perfect, objective solution we'd all like it to be - but it's a damn sight more objective than not doing it at all.

In short, reviewers :
  • Specify what it is you want the authors to do. Authors aren't psychic. They'd like to be, but they're not. This means vague instructions really aren't helpful. Asking the author to "do this bit again, but better" (that's a real example, albeit paraphrased) won't work !
  • Don't make non-constructive criticisms. Ask questions rather than stating that the authors have missed something obvious - it at least gives the impression of benefit of the doubt. There's no point whinging that the paper has lots of typos - send them a list instead*. As above, vague, hand-waving instructions help no-one.
  • Don't move the goalposts. Spend longer on the first draft so that the authors will be able to address as much as possible at stage one. Telling them that they need minor revisions at the first draft, then saying major revisions at the second draft (after the previous comments have been addressed) is misleading, unprofessional and delays publication. And if you do find a major error at the second draft, have the common courtesy to apologise for not spotting it sooner !
  • Demand rigour, but don't be anal about it. Life's too short to worry about half-spaces, or, more importantly, the precise meaning of a word. Insisting that the author uses a long-winded phrase (e.g. "galaxies in the overdensity between redshift 0.02 and 0.03") just in case a commonly-used word (e.g. "population" - seriously, referee 1.0, WTF have you got against the word "population" ?) is misunderstood helps no-one.
  • Check that the author hasn't already addressed your point in the paper or their response. There's really no reason for the author to bother if you're not going to read what they write - it makes the whole process meaningless and unworkable.
  • Remember that not everyone is a native English speaker. Many of my non-Anglo/American colleagues report that referees can be very unhelpful when it should be obvious enough that the author might not speak English as their first language !** If you suspect this is the case, then criticising their writing flaws is simply rude. Don't do it. Instead, give them a list of corrections.
  • If you can't justify your assertions, don't make them. The same applies to the authors, of course. Unless you can point out why something is wrong - and, if at all possible, how to correct it - don't say what you think is wrong. If the authors have justified what they've done, you may disagree with them - but unless you can prove that they're wrong, that should be the end of it. You've got to allow people to publish things you disagree with.
* And no, they can't be "easily avoided." If you've read your own work a dozen times it becomes literally impossible to spot any more typos. Fresh eyes are needed, and it makes a lot of sense to me that those eyes should be provided by the referee/editor. To say, "please check more carefully in future" is condescending.
**Helpful hint : if the author's name is Pierre von Hindenberg, chances are they're not a native English speaker - and with a name like that, they're probably way more awesome than you will ever be.