Perhaps you are familiar with the following passage from Bertrand Russell:
“Observation versus Authority: To modern educated people, it seems obvious that matters of fact are to be ascertained by observation, not by consulting ancient authorities. But this is an entirely modern conception, which hardly existed before the seventeenth century. Aristotle maintained that women have fewer teeth than men; although he was twice married, it never occurred to him to verify this statement by examining his wives’ mouths.” 
This criticism of Aristotle is often repeated and unreflectively accepted due to the reputation of Bertrand Russell. Edward de Bono embroidered upon this theme:
“Finally there was Aristotle, with his word-based inclusion/exclusion logic. Aristotle believed that men had more teeth than did women. Although he was married twice, he never actually counted the teeth of either wife. He did not need to. With horses, the stallion had more teeth than the mare; so he “knew” that the male of the species has more teeth than the female. Aristotle derived his categories from the past and then argued whether something did or did not fit into a particular category.” 
It is useful to look at what Aristotle actually said:
”Males have more teeth than females in the case of men, sheep, goats, and swine; in the case of other animals observations have not yet been made.” 
Of course Aristotle is wrong, but he is wrong because he was misinformed about the observation, not — as Russell and de Bono suggest, because he did not believe that observations were important. Aristotle believed that observation was important and said so. He is claiming that his knowledge derives from observation and he is carefully listing only the species in which he believes that this observation has been made. His use of the word “yet” demonstrates that he believed more observations were necessary.
Much of Aristotle’s writings on animals uses first hand observation, but he could not possibly make personal observations of every animal on which he wrote and so had to rely on observations that were made by others whom he trusted. Any scientist working today does exactly the same, but has the luxury of multiple sources at his disposal, which Aristotle did not have.
Bertrand Russell was married three times but does not disclose whether or not he counted his wives’ teeth in order to ascertain that Aristotle was wrong. I imagine that, like Aristotle, his belief about this is formed by trusting an observation that has already been made, the difference being that his information, mid-twentieth century, is better than that available in the 4th century BCE.
Notwithstanding this advantage, Russell still manages to be wrong about Aristotle’s attitude toward observation. While Russell castigates Aristotle for not counting his wives’ teeth, it does not appear to have occurred to Russell to verify his own statement by going to the bookshelf and reading what Aristotle actually wrote.
Aristotle seems to be popular as a whipping boy these days, a sport often based upon myths like the one above. The more of these myths we hear repeated, the more people are inclined to believe that Aristotle held absurd opinions. The Wikipedia entry on the history of pain theory , for instance, says “Aristotle believed that pain was due to evil spirits that entered the body through an injury” and cites not a primary text by Aristotle, but “Models of Pain Perception”  by Steven Linton as the source. The author of that text says: “Aristotle, on the other hand, asserted that pain was due to evil spirits and the gods. These entered the body via an injury.”
Anyone who is even slightly familiar with Aristotle will be more than a little suspicious of that — it doesn’t sound like anything that Aristotle would say. Turns out that Linton uses as his source David Morris’ “Sociocultural and Religious Meanings of Pain” . However, Linton appears to have misread Morris, who does not claim this, but rather mentions that some cultures believed evil spirits caused pain in a paragraph following one in which Aristotle is mentioned.
Thus by Chinese Whispers the claim gains spurious authority in Wikipedia, and the importance of citing primary texts is demonstrated once again.
Or take Stephen Hawking and Leonard Mlodinov. In their book “The Grand Design” , they write:
“To explain the fact that objects clearly pick up speed as they fall, he [Aristotle] invented a new principle — that bodies proceed more jubilantly, and hence accelrate, when they come closer to their natural place of rest, a principle that today seems a more apt description of certain people than of inanimate objects.”
Again, to anyone who is even a little bit familiar with Aristotle, this claim seems suspect. It doesn’t sound like the kind of thing that Aristotle said, and the authors do not cite where this notion comes from. After a little digging I found that the Stanford Encyclopedia of Philosophy has identified the source of this claim as the psychologist B F Skinner. The commentary is scathing:
“Indeed, it is evident that whatever the merits of the most penetrating of such criticisms, much of the contumely directed at Aristotle is stunningly illiterate. To take but one of any number of mind-numbing examples, the famous American psychologist B.F. Skinner reveals that ‘Aristotle argued that a falling body accelerated because it grew more jubilant as it found itself nearer its home.’ To anyone who has actually read Aristotle, it is unsurprising that this ascription comes without an accompanying textual citation. For Aristotle, as Skinner would portray him, rocks are conscious beings having end states which they so delight in procuring that they accelerate themselves in exaltation as they grow ever closer to attaining them. There is no excuse for this sort of intellectual slovenliness…” 
That was not difficult to find, but Mlodinov and Hawking, so careful with evidence when they are doing physics, seem to have uncritically believed this claim rather than checking it.
What I find astonishing is that, while a non academic like me with only a passing knowledge of Aristotle can read that and immediately spot that there is something wrong with the claim, the authors, not to mention the editors, sub-editors and proof-readers whose hands these books would also pass through, did not have even the sparse knowledge of Aristotle that I have. And yet this book and the authors’ reputations will likely stand as authority in the future on behalf of this false claim.
By far the most common example of Aristotle getting something wrong is that he believed that heavier bodies fall quicker than lighter bodies. It is quite difficult to track down where Aristotle said this as the claim is so familiar it is always uncited. It seems likely that for most people the source of this claim is Galileo’s “A Dialogue Concerning Two New Sciences” in which he attributes words to Aristotle which the philosopher seems never to have said. I could find two places where Aristotle has made a similar claim and one is in “On the Heavens”:
“Again, the suggestion of a certain ratio between the void and the solid in a body is no more equal to solving the problem before us. The manner of speaking will issue in a similar impossibility. For any two portions of fire, small or great, will exhibit the same ratio of solid to void, but the upward movement of the greater is quicker than that of the less, just as the downward movement of a mass of gold or lead, or of any other body endowed with weight, is quicker in proportion to its size. This, however, should not be the case if the ratio is the ground of distinction between heavy things and light.” 
Yes, Aristotle got it wrong. However, again, his failure is due to an incorrect observation rather than a faulty methodology. Aristotle is saying that a particular hypothesis is wrong because it is ruled out by an observation, so his approach was right on the principle, he was just wrong about the specific observation.
Perhaps the most fashionable criticism of Aristotle is that he believed that logic and not observation was the way to acquire knowledge about the world, as Lawrence Krauss puts it:
“But the point is, if we continue to rely on our understanding of the universe on Aristotelian logic, on classical logic, by what we think is sensible, we would still be living in a world where heavier objects, we think, fall faster than light objects, because they’re heavier, as Aristotle used to think, instead of doing the experiment to check it out.” 
The problem is that, as we have seen, Aristotle did not arrive to his conclusion through logic, but via faulty data. Moreover, Aristotle explicitly rejected the idea that logic can be used to gain knowledge about the world and insisted instead that we must start with an inductive process based on empirical data . So the point that Krauss is making is exactly the same point that Aristotle made two thousand, three hundred years ago, and yet Krauss phrases it as a criticism of the Aristotelian approach!
I am not saying that Aristotle invented anything like the modern scientific method. He struggled to find the best mix of observation and deductive logic, and he did not employ the mathematical approach that Archimedes and Euclid would develop a few decades later. But we cannot read Aristotle as though he was writing yesterday, or a century ago or even five centuries ago. It does not diminish Aristotle’s stature that, in the fourth century BCE, one man in one lifetime did not complete what it would take the rest of humankind another 2,000 years to achieve.
Edward de Bono complained that Aristotle sent us in the wrong direction and that we would have achieved more, sooner if Aristotle had not existed. But de Bono is basing this opinion on factually incorrect information about what Aristotle wrote. It is de Bono, not Aristotle, who is tailoring his evidence to his conclusion.
Aristotle said much that has turned out to be wrong. As far as some of his errors are concerned, it may seem to us absurd that an intelligent man could have made them in any historical time, and yet it is certainly difficult to know what might and might not have seemed reasonable 2,300 years ago. Even today most of what we believe we have not tested for ourselves, and we trust a number of sources instead, just like Aristotle did. We do not know much about his sources either; for example, he may have heard information on the care and feeding of elephants from mahouts and thought it reasonable to trust them.
If you are still inclined to judge him harshly, consider what nonsense Hawking, Mlodinov, de Bono, Skinner, and a long list of others believe about Aristotle without a shred of evidence, and how many people in turn believe these authors, even though our sources of evidence today are far more numerous, more reliable, and more easily accessible than anything Aristotle had. Even today highly intelligent people can believe nonsensical notions without evidence, and none of us can claim to be immune from the problem.
Moreover, there is much in Aristotle’s writings that demonstrates careful observation. Aristotle’s description of a bird at various stages of its development can only have come from Aristotle himself having done the dissections of eggs at various stages of development or at least having been present at such a dissection. It is impressive work even today. 
The knee-jerk reaction to dismiss Aristotle is, I think, due to the fact that he was so enthusiastically adopted by the Scholastics in the middle ages. This is, in itself, a misstatement of the actual history of ideas, but even if it were the case, we can of course hardly blame Aristotle for how people interpreted him, often on the basis of partial texts, so long after his death.
It is time we revive Aristotle and rescue him from common slanders and misinformation. His achievements were impressive, and include setting us on a course for the development of the modern scientific method — through his systemization of logic and by the demonstration that logic alone cannot provide us with scientific knowledge about the world (contra, for example, what Plato — Aristotle’s teacher — thought). He was, as Kant describes him, “the Bede of the Empiricists.”
Those who advocate a scientific world view should, in fact, cherish Aristotle, recognising him as a pioneer. Think of him not as “The Philosopher” of Aquinas’s Summa, but as a man who can justifiably be considered the first biologist.
At the very least, next time you read or hear something ludicrous that Aristotle is supposed to have said or believed, don’t trust it unless you can find a primary text in which Aristotle actually said it. Evidence before credence.
Robin Herbert is a software developer from Sydney, as well as an intermittent student of mathematics with a long time interest in the history and philosophy of science.
 Bertrand Russell, The Impact of Science on Society, p. 7. Simon & Schuster Inc, 1968.
 Edward de Bono, Guardian, 25 January 1997.
 Aristotle, On the Parts of Animals: Book III.
 Wikipedia on the history of pain.
 Steven Linton, Models of Pain Perception, Elsevier Health, 2005.
 David Morris, Sociocultural and Religious Meanings of Pain.
 Stephen Hawking and Leonard Mlodinov, “The Grand Design,” Bantam, 2010.
 Stanford Encyclopedia of Philosophy on Aristotle.
 Aristotle, On the Heavens, Book IV, part 2.
 Lawrence Krauss vs William Lane Craig debate, North Carolina State University, March 30 2011.
 Aristotle, Posterior Analytics, Book II, part 19.
 Aristotle, History of Animals, Book IV, part 3.