To be able to assess animal welfare the researcher must presuppose a number of background assumptions that cannot be tested by means of ordinary empirical data collection. In order to substantiate these assumptions two sorts of inferences have to be relied upon, which the authors designate by the terms ‘analogies’ and ‘homologies’. Analogies are evaluative, philosophical reflections by means of which it is made clear what provisions or states constitute the welfare of humans and other animals. By means of analogies it may, for example be argued that animal welfare consists of subjective experiences such as pain, boredom, pleasure and expectation. Also by means of analogies the relative ‘weight’ of these states can be decided. Homologies are part of theoretical science. They serve to clarify how the relevant experiences are linked to measurable anatomical, physiological and behavioural parameters.

An account is given of the steps which have to be taken to give a full answer to a question concerning the welfare of animals. In the account only farm animals are mentioned, but the same steps, of course, also have to be taken to answer questions concerning the welfare of other kinds of animals be they companion, laboratory, zoo or wild. Eight steps are described, and it is argued that both analogies and homologies are needed at very fundamental levels. Therefore, if animal welfare science is to provide relevant, rational and reliable answers to questions concerning animal welfare, it must be an interdisciplinary inquiry involving philosophical reflections and theoretical biology.

Scientific research on ‘animal welfare’ began because of ethical concerns over the quality of life of animals, and the public looks to animal welfare research for guidance regarding these concerns. The conception of animal welfare used by scientists must relate closely to these ethical concerns if the orientation of the research and the interpretation of the findings is to address them successfully.

At least three overlapping ethical concerns are commonly expressed regarding the quality of life of animals: (1) that animals should lead natural lives through the development and use of their natural adaptations and capabilities, (2) that animals should feel well by being free from prolonged and intense fear, pain, and other negative states, and by experiencing normal pleasures, and (3) that animals should function well, in the sense of satisfactory health, growth and normal functioning of physiological and behavioural systems. Various scientists have proposed restricted conceptions of animal welfare that relate to only one or other of these three concerns. Some such conceptions are based on value positions about what is truly important for the quality of life of animals or about the nature of human responsibility for animals in their care. Others are operational claims: (1) that animal welfare research must focus on the functioning of animals because subjective experiences fall outside the realm of scientific enquiry, or (2) that studying the functioning of animals is sufficient because subjective experiences and functioning are closely correlated. We argue that none of these positions provides fully satisfactory guidance for animal welfare research.

We suggest instead that ethical concerns about the quality of life of animals can be better captured by recognizing three classes of problems that may arise when the adaptations possessed by an animal do not fully correspond to the challenges posed by its current environment. (I) If animals possess adaptations that no longer serve a significant function in the new environment, then unpleasant subjective experiences may arise, yet these may not be accompanied by significant disruption to biological functioning. Thus, a bucket-fed calf may experience a strong, frustrated desire to suck, even though it obtains adequate milk. (2) If the environment poses challenges for which the animal has no corresponding adaptation, then functional problems may arise, yet these may not be accompanied by significant effects on subjective feelings. Thus, a pig breathing polluted air may develop lung damage without appearing to notice or mind the problem. (3) Where animals have adaptations corresponding to the kinds of environmental challenges they face, problems may still arise if the adaptations prove inadequate. For example, an animal's thermoregulatory adaptations may be insufficient in a very cold environment such that the animal both feels poorly and functions poorly. We propose that all three types of problems are causes of ethical concern over the quality of life of animals and that they together define the subject matter of animal welfare science.