In the modern era the French philosopher Rene Descartes and his pupil Nicolas Malebranche propounded the view that animals felt neither joy nor pain. Descartes was of the opinion that all animal behaviour - like the actions of a robot - could be attributed to reflexes. Unlike humans, he believed that animals did not have a soul (res cogitans) that would enable them to feel and think, but merely a material body (res extensa) that reacted mechanically to stimuli.

The most dramatic implication was that animal cries of pain were not an expression of experienced suffering, but merely unconscious reflexes ("like the squeaking of a door"). In the 1970s the view that nothing scientific could be said about an animal's experience become topical again due to the emergence of "behaviourism".

At the same time there have always been tendencies - especially in folk psychology - to understand animal behaviour by analogy to human behaviour and attribute it to complex mental states (for example desires and convictions); such an interpretation is generally referred to as "anthropomorphic".

In search of a middle way between Descartes' denial of an animal soul and the anthropomorphism of folk psychology, cognitive ethology explores animal thought and experience. Such research also has a bearing on the issue of the ethical evaluation of animal experiments:

(1) an ethical problem with animal experiments would only exist at all (as it is generally believed) if animals were capable of feeling. Yet even if an animal's capacity to feel pain is accepted in principle, the further question arises as to whether the mental trauma of test animals should be considered greater or less than the mental trauma of humans in a comparable situation on account of an animal's lack of self and future awareness.

(2) The minimisation of animal suffering - as is required by law - is contingent upon a reliable assessment of what the animal is feeling. Currently, however, there is no consensus on reliable criteria for the attribution of pain. The wordings of directives, laws etc. therefore often require that animals' sensitivity to pain should be assumed to be analogous to that of humans, although there is no evidence that all living creatures respond to the same stimuli with pain.