Why is Bertrand Russell so critical of Aristotle and Aristotelian logic?

“Any person in the present day who wishes to learn logic will be wasting his time if he reads Aristotle or any of his disciples. If you wish to become a logician, there is one piece of advice which I cannot urge too strongly, and that is: Do NOT learn the traditional formal [Aristotelian] logic ... I conclude that the Aristotelian doctrines are wholly false, with the exception of the formal theory of the syllogism, which is unimportant ... Aristotle's merits are enormous; in the latter, his demerits are equally enormous. For his demerits, however, his successors are more responsible than he is.“

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“Logic was practically invented by Aristotle. For nearly two thousand years, his authority in logic was unquestioned. To this day, teachers in Catholic educational institutions are not allowed to admit that his logic has defects, and any non-Catholic who criticizes it incurs the bitter hostility of the Roman Church. Undue respect for Aristotle, however, is not confined to Catholic institutions. In most universities, the beginner in logic is still taught the doctrine of the syllogism, which is useless and complicated, and an obstacle to a sound understanding of logic. If you wish to become a logician, there is one piece of advice which I cannot urge too strongly, and that is: Do NOT learn the traditional formal logic. In Aristotle’s day it was a creditable effort, but so was the Ptolemaic astronomy. To teach either in the present day is a ridiculous piece of antiquarianism.“

— Bertrand Russell, The Art of Philosophizing and Other Essays (1943), Essay III: The Art of Drawing Inferences, p. 38

Russell continues:

“Aristotle's influence, which was very great in many different fields, was greatest of all in logic. In reading any important philosopher, but most of all in reading Aristotle, it is necessary to study him in two ways: with reference to his predecessors, and with reference to his successors. In the former aspect, Aristotle's merits are enormous; in the latter, his demerits are equally enormous. For his demerits, however, his successors are more responsible than he is.

... Aristotle, as a philosopher, is in many ways very different from all his predecessors. He is the first to write like a professor: his treatises are systematic, his discussions are divided into heads, he is a professional teacher, not an inspired prophet. His work is critical, careful, pedestrian, without any trace of Bacchic enthusiasm. The Orphic elements in Plato are watered down in Aristotle, and mixed with a strong dose of common sense; where he is Platonic, one feels that his natural temperament has been overpowered by the teaching to which he has been subjected.

He is not passionate, or in any profound sense religious. The errors of his predecessors were the glorious errors of youth attempting the impossible; his errors are those of age which cannot free itself of habitual prejudices. He is best in detail and in criticism; he fails in large construction, for lack of fundamental clarity and Titanic fire.

... His present-day influence is so inimical to clear thinking that it is hard to remember how great an advance he made upon all his predecessors (including Plato), or how admirable his logical work would still seem if it had been a stage in a continual progress, instead of being (as in fact it was) a dead end, followed by over two thousand years of stagnation. In dealing with the predecessors of Aristotle, it is not necessary to remind the reader that they are not verbally inspired; one can therefore praise them for their ability without being supposed to subscribe to all their doctrines. Aristotle, on the contrary, is still, especially in logic, a battle-ground, and cannot be treated in a purely historical spirit.

I conclude that the Aristotelian doctrines with which we have been concerned in this chapter are wholly false, with the exception of the formal theory of the syllogism, which is unimportant. Any person in the present day who wishes to learn logic will be wasting his time if he reads Aristotle or any of his disciples.

None the less, Aristotle's logical writings show great ability, and would have been useful to mankind if they had appeared at a time when intellectual originality was still active. Unfortunately, they appeared at the very end of the creative period of Greek thought, and therefore came to be accepted as authoritative. By the time that logical orginality revived, a reign of two thousand years had made Aristotle very difficult to dethrone. Throughout modern times, practically every advance in science, in logic, or in philosophy has had to be made in the teeth of the opposition from Aristotle's disciples.“

— Bertrand Russell, A History of Western Philosophy (1945), Book One. Ancient Philosophy, Part II. Socrates, Plato and Aristotle, Ch: XXII, pp. 202-3 & 200

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Background: Aristotle (384–322 BCE)

Aristotle was an Ancient Greek philosopher and polymath. He began the wider Aristotelian tradition that followed, which set the groundwork for the development of logic and modern science. Aristotle contributed to almost every field of human knowledge then in existence. Aristotle's logic greatly influenced the history of Western thought. It was Aristotle's logic which was copied and used in the Arabic and Latin medieval traditions. It was dominant for two and a half thousand years, until the late 19th century. Then modern logic was started by Gottlob Frege, Charles Sanders Peirce and others. Though Aristotle wrote many elegant treatises and dialogues for publication, only around a third of his original output has survived. Aristotelian logic was highly regarded in the Hellenized world, especially in Alexandria and then throughout the Roman Empire. In medieval times Aristotle was revered, both in the Christian and Islamic worlds, to the point where Saint Thomas Aquinas called him simply "THE Philosopher".

Aristotle's influence on logic continued well into the 19th century. The 19th-century German philosopher Friedrich Nietzsche has been said to have taken nearly all of his political philosophy from Aristotle. However, Nietzsche admired the pre-Socratics deeply, calling them "tyrants of the spirit" to mark their antithesis and his preference against Socrates and his successors such as Plato and Aristotle. Charles Darwin regarded, not himself, but Aristotle as the most important contributor to the subject of biology. Darwin personally believed evolutionary ideas could be traced to Aristotle. Karl Marx considered Aristotle to be the "greatest thinker of antiquity" and called him "a genius".

In the 20th century, the great portion of Aristotle's work was heavily criticized. This movement was primarily led by philosopher Bertrand Russell and other analytic philosophers. Russell's famous student, Ludwig Wittgenstein, boosted proudly to Russell that he never read Aristotle. However, Wittgenstein was notorious for being rather poorly read in the history of philosophy. Russell argued that "almost every serious intellectual advance has had to begin with an attack on some Aristotelian doctrine". Russell called Aristotle's ethics "repulsive" and labelled his logic "as definitely antiquated as Ptolemaic astronomy". Russell stated that these errors made it difficult to do historical justice to Aristotle, until one remembered what an advance he made upon all of his predecessors. More than 2300 years after his death, it must be admitted, as Russell reluctantly does in the above quotes, Aristotle remains one of the most influential, although not always "accurate thinkers" who ever lived.

Image: Bust of Aristotle. Marble, Roman copy after a Greek bronze original by Lysippos from 330 BC; the alabaster mantle is a modern addition.