The scientific works to which Aristotle’s name is attached may be divided into three groups, physical, biological, and psychological. In size they vary from such a large treatise as the History of Animals to the tiny tracts which go to make up the Parva naturalia. So far as the scientific writings can be distinguished as separate works they may be set forth as follows:

Physics.
φυσικὴ ἀκρόασιςPhysics.
περὶ γενέσεως καὶ φθορᾱςOn coming into being and passing away.
περὶ οὐρανοῡ.On the heavens.
μετεωρολογικά.Meteorology.
[περὶ κόσμου.On the universe.]
[μηχανικά.Mechanics.]
[περὶ ἀτόμων γραμμῶν.On indivisible lines.]
[ἀνέμων θέσεις καὶ προσηγορίαι.Positions and descriptions of winds.]
Biology in the restricted sense.
(a) Natural History.
περὶ τὰ ζῳα ἱστορίαι.Inquiry about animals = Historia animalium.
περὶ ζῴων μορίων.On parts of animals.
περὶ ζῴων γενέσεως.On generation of animals.
[περὶ φυτῶν.On plants.]
(b) Physiology.
περὶ ζῴων πορείας.On progressive motion of animals.
περὶ μακροβιότητος καὶ βραχυβιότητος.On length and shortness of life.
περὶ ἀναπνοῆς.On respiration.
περὶ νεότητος καὶ γήρως.On youth and age.
[περὶ ζῴων κινήσεως.On motion of animals.]
[φυσιογνωμονικά.On physiognomy.]
[περὶ πνεύματος.On innate spirit.]
Psychology and Philosophy with biological bearing.
περὶ ψυχῆς.On soul.
περὶ αἰσθήσεως καὶ αἰσθητῶν.On sense and objects of sense.
περὶ ζωῆς καὶ θανάτου.On life and death.
περὶ μνήμης καὶ ἀναμνήσεως.On memory and reminiscence.
περὶ ὓπνου καὶ ἐγρηγόρσεως.On sleep and waking.
περὶ ἐνυπνίων.On dreams.
[προβλήματα.Problems.]
[περὶ χρωμάτων.On colours.]
[περὶ ἀκουστῶν.On sounds.]
[περὶ τῆς καθ’ ὔπνον μαντικῆς.On prophecy in sleep.]

Of these works some, the names of which are placed here in brackets, are clearly spurious in that they were neither written by Aristotle nor are they in any form approaching that in which they were cast by him. Yet all are of very considerable antiquity and contain fragments of his tradition in a state of greater or less corruption. In addition to works here enumerated there are many others which are spurious in a yet further sense in that they are merely fathered on Aristotle and contain no trace of his spirit or method. Such, for example, is the famous mediaeval work of oriental origin known as the Epistle of Aristotle to Alexander.

In a general way it may be stated that the physical works, with which we are not here directly concerned, while they show ingenuity, learning, and philosophical power, yet betray very little direct and original observation. They have exerted enormous influence in the past and for at least two thousand years provided the usual physical conceptions of the civilized world both East and West. After the Galilean revolution in physics, however, they became less regarded and they are not now highly esteemed by men of science. The biological works of Aristotle, on the other hand, excited comparatively little interest during the Middle Ages, but from the sixteenth century on they have been very closely studied by naturalists. From the beginning of the nineteenth century, and especially as a result of the work of Cuvier, Richard Owen, and Johannes Müller, Aristotle’s reputation as a naturalist has risen steadily, and he is now universally admitted to have been one of the very greatest investigators of living nature.

The philosophical bases of Aristotle’s biology are mainly to be found in the treatise On soul and in that On the generation of animals. His actual observations are contained in this latter work—which is in many ways his finest scientific production—in the great collection on the History of animals, and in the remarkable treatise On parts of animals. Certain of his deductions concerning the nature and mechanism of life can be found in his two works which deal with the movements of animals (one of which is very doubtfully genuine) and in his tracts On respiration, On sleep, &c. The treatise On plants and the Problems in their present form are late and spurious, but they are based on works of members of his school. They were, however, perhaps originally prepared at the other end of the Greek world in Magna Graecia.

Aristotle was a most voluminous author and his biological writings form but a small fraction of those to which his name is attached. Yet these biological works contain a prodigious number of first-hand observations and it has always been difficult to understand how one investigator could collect all these facts, however rapid his work and skilful his methods. The explanations that have reached us from antiquity are, indeed, picturesque, but they are neither credible in themselves nor are they consistent with each other. Thus Pliny writing about a. d. 77 says ‘Alexander the Great, fired by desire to learn of the natures of animals, entrusted the prosecution of this design to Aristotle.... For this end he placed at his disposal some thousands of men in every part of Asia and Greece, and among them hunters, fowlers, fishers, park-keepers, herds-men, bee-wards, as well as keepers of fish-ponds and aviaries in order that no creature might escape his notice. Through the information thus collected he was able to compose some fifty volumes.’[14] Athenaeus, who lived in the early part of the third century a. d., assures us that ‘Aristotle the Stagirite received eight hundred talents [i.e. equal to about £200,000 of our money] from Alexander as his contribution towards perfecting his History of Animals’.[15] Aelian, on the other hand, who lived at a period a little anterior to Athenaeus, tells us that it was ‘Philip of Macedon who so esteemed learning that he supplied Aristotle with ample funds’ adding that he similarly honoured both Plato and Theophrastus.[16]

Now in all Aristotle’s works there is not a single sentence in praise of Alexander and there is some evidence that the two had become estranged. In support of this we may quote Plutarch (c. a. d. 100) who gives a detailed description of a conspiracy in 327 b. c. against Alexander by Callisthenes, a pupil of Aristotle who appears to have kept up a correspondence with his master.[17] Alexander himself wrote of Callisthenes, according to Plutarch: ‘I will punish this sophist, together with those who sent him to me and those who harbour in their cities men who conspire against my life’ and Plutarch adds that Alexander ‘directly reveals in these words a hostility to Aristotle in whose house Callisthenes ... had been reared, being a son of Hero who was a niece of Aristotle’.[18] Yet the Alexandrian conquests, bringing Greece into closer contact with a wider world and extending Greek knowledge of the Orient, must have had their influence in stimulating interest in rare and curious creatures and in a general extension of natural knowledge. That the interest in these topics extended beyond the circle of the Peripatetics is shown by the fact that Speusippus, the pupil of Plato and his successor as leader of his school, occupied himself with natural history and wrote works on biological topics and especially on fish.

Nevertheless, remarkable as is Aristotle’s acquaintance with animal forms, investigation shows that he is reliable only when treating of creatures native to the Aegean basin. As soon as he gets outside that area his statements are almost always founded on hearsay or even on fable.[19] Whatever assistance Aristotle may have received in the preparation of his biological works came, therefore, probably from no such picturesque and distant source as the gossip of Pliny or Aelian would suggest. We can conjecture that he received aid from the powerful relatives of his wife at Atarneus and in Lesbos, and we may most reasonably suppose that after his return to Athens much help would have been given him by his pupils within the Lyceum. To them may probably be ascribed many passages in the biological writings; for it seems hardly possible that Aristotle himself would have had time for detailed biological research after he had settled as a teacher in Athens. Of the work of these members of his school a fine monument has survived in two complete botanical treatises and fragments of others on zoological and psychological subjects by Theophrastus of Eresus, his pupil and successor in the leadership of the Lyceum and perhaps his literary legatee.

When we turn to the Aristotelian biological works themselves we naturally inquire first into the question of genuineness, and here a difficulty arises in that all his extant works have come down to us in a state that is not comparable to those of any other great writer. Among the ancients admiration was expressed for Aristotle’s eloquence and literary powers, but, in the material that we have here to consider, very little trace of these qualities can be detected by even the most lenient judge. The arrangement of the subject-matter is far from perfect even if we allow for the gaps and disturbances caused by their passage through many hands. Moreover, there is much repetition and often irrelevant digression, while the language is usually plain to baldness and very frequently obscure. We find sometimes the lightening touch of humour, but the style hardly ever rises to beauty. Furthermore, even in matters of fact, while many observations exhibit wonderful insight and, forestalling modern discovery, betray a most searching and careful application of scientific methods, yet elsewhere we find errors that are childish and could have been avoided by the merest tyro.

This curious state of the Aristotelian writings has given rise to much discussion among scholars and to explain it there has been developed what is known as the ‘notebook theory’. It is supposed that the bases of the material that we possess were notebooks put together by Aristotle himself for his own use, probably while lecturing. These passed, it is believed, into the hands of certain of his pupils and were perhaps in places incomprehensible as they stood. Such pupils, after the master’s death, filled out the notebooks either from the memory of his teaching or from their own knowledge—or ignorance. Thus modified, however, they were still not prepared for publication, even in the limited sense in which works may be said to have been published in those days, but they formed again the fuller bases of notes for lectures delivered by his successors. In this form they have finally survived to our time, suffering, however, from certain further losses and displacements on a larger scale. Some of the ‘Aristotelian’ works are undoubtedly more deeply spurious, but the works that are regarded as ‘genuine’ do not seem to have been seriously tampered with, except by mere scribal or bookbinders’ blunders, at any date later than a generation or two following Aristotle’s own time. These notebooks as they stand are in fact probably in much the state in which we should find them were we able to retrieve a copy dating from the first or second century b. c.[20]