When assessing discomfort in animals analogous reasoning is often used, namely, that the causes or symptoms of discomfort in people will also apply to animals. This practical rule of thumb can be based on an ‘analogy-postulate’. This postulate takes into account the anatomical and physiological similarities of vertebrate nervous systems and the comparability and homology in the behavioural and physiological responses to discomfort of humans and other vertebrates in similar situations. There are theoretical and practical problems with this analogous reasoning. Theoretical objections include claims that feelings do not exist, are irrelevant or that scientific knowledge is not necessary to recognize feelings. Practical problems will occur when assessing the discomfort of animals without proper knowledge of the relevant species-specific information. Nevertheless, we think that there are two equivalent sound reasons for accepting the analogy-postulate. First, there is more evidence in favour of acceptance of the postulate than of its rejection. Secondly, the negative moral consequences of erroneously rejecting the postulate are far greater than those of mistakenly accepting it.
