In the philosophy of mind as well as ethics the treatment of animals exposes major problems. If other animals differ from human beings, how is the difference to be characterized: do animals think and reason, or have thoughts and beliefs? In philosophers as different as Aristotle and Kant the possession of reason separates humans from animals, and also alone allows entry to the moral community. For Descartes, animals are mere machines, and even lack consciousness or feeling. In the ancient world the rationality of animals is defended with the example of Chrysippus' dog. This animal, tracking a prey, comes to a crossroads with three exits, and after sniffing two of them and failing to find the scent, dashes down the third without pausing to pick up the scent, reasoning, according to Sextus Empiricus: ‘the animal went either by this road, or by that, or by the other; but it did not go by this or that, therefore he went the other way.’ The ‘syllogism of the dog’ was discussed by many writers, since in Stoic cosmology animals should occupy a place on the great chain of being somewhere below human beings, the only terrestrial rational agents. Philo Judaeus wrote a dialogue attempting to show against Alexander of Aphrodisias that the dog's behaviour does not exhibit rationality, but simply shows it following the scent; by way of response Alexander has the animal jump down a shaft (where the scent would not have lingered). Plutarch sides with Philo. Aquinas discusses the dog (Summa Theologiae, IaIIae 13. 2, 3), and scholastic thought in general was quite favourable to brute intelligence (it was not uncommon for animals to be made to stand trial for various offences in medieval times). In the modern era Montaigne uses the dog to remind us of the frailties of human reason; Rorarius undertook to show not only that beasts are rational, but that they make better use of reason than people do. James I of England defends the syllogizing dog, and Henry More and Gassendi both take issue with Descartes on the matter. Hume is an outspoken defender of animal cognition, but with the rise of the view that language is the essential manifestation of mentality, animals' silence began to count heavily against them, and they are completely denied thoughts by, for instance, Davidson. Dogs are frequently shown in pictures of philosophers, as their assiduity and fidelity is a symbol of what is needed in the hunt for wisdom. See also instinct.