Similarity Arguments
If we do not, after all, "just know" that most animals possess phenomenal consciousness, then it seems we must infer its presence or absence. Similarity arguments invoke some human or mechanical analogue to explain the behaviour of non-human animals, and use the similarities between the analogue's features and those of non-human animals to justify their conclusions about animal awareness, while dissimilarity arguments do the opposite and argue from disanalogies (Allen, 2002).
Similarity arguments for animal awareness in non-human animals use human beings as the analogue, and trade on the numerous resemblances between our anatomy, behaviour, neurobiology and pharmacology and theirs. Panksepp (2002b) invokes such an argument when summarising the available evidence for affective consciousness in non-human animals:
Overwhelming evidence shows that animal brains elaborate many states of affective consciousness... Of course there is no "ultimate "proof" in science, merely the weight of evidence. To me it remains a mystery that certain scientists can ignore the mass of relevant evidence from (i) behavioral reinforcement studies; (ii) place preference-aversion studies; (iii) manifest and ubiquitous emotional vocalizations; (iv) neuroethological studies evoking the same emotional behavior from the same human/animal brain analogs and (v) the coherent translations between human and animal psychopharmacological work.
Likewise, Baars (2001), reviewing the neural and behavioural evidence for consciousness in animals, writes:
The brain and behavioral evidence for subjective consciousness is essentially identical in humans and other mammals... It seems that the burden of proof for the absence of subjectivity in mammals should be placed on the skeptics.
Similarity arguments against animal consciousness point to similarities between animal behaviour and that of machines or human individuals that lack awareness. For instance, Rose (2002) argues that the complex nociceptive behaviour exhibited by animals (e.g. fish) when evading noxious stimuli does not indicate conscious pain on their part, as similar behaviour can be found in patients in a permanent vegetative state.
Dissimilarity arguments are usually cited to prove that other animals do not possess awareness as we do. These arguments may be based on disanalogies between the behaviour of conscious human beings and that of certain animals, who are then said to lack awareness; or on the absence of some mental attribute in non-human animals, which is then alleged to be a pre-requisite for consciousness. The best-known philosophical exponent of this strategy is Carruthers (1998, 2000) who refuses to ascribe phenomenally conscious feelings to non-human animals - except maybe chimpanzees - because they lack a "theory of mind". (We shall examine the evidence for animals having a "theory of mind" later in this chapter.)
Rather than address similarity and dissimilarity arguments in detail, I propose to step back and examine their inherent limitations. Similarity arguments that make use of some (human or mechanical) analogue work straightforwardly if all of the similarities between animals and the analogue are relevant to the capacity for consciousness, and none of the dissimilarities are relevant.
Similarity arguments run into difficulties when we find both relevant similarities and relevant dissimilarities. If the similarities and dissimilarities relate to the same kind of evidence, one usually evaluates the evidence in its totality. The metaphors of "stepping back to look at the "big picture" and "weighing up the pluses and minuses" are sometimes used to describe this kind of evaluative procedure, which can be carried out satisfactorily if the comparisons are performed systematically.
However, similarity arguments break down when the similarities and dissimilarities relate to two completely kinds of evidence. In this case, the similarities and dissimilarities can no longer be "weighed up" against each other and are thus incommensurable.
This is precisely what I found in my search of the literature relating to the capacity of non-mammals to experience pleasure and pain. On the one hand, there are strong behavioural similarities (discussed below) between mammals (including humans), birds and reptiles, suggesting that all of these animals have conscious feelings. On the other hand, the neurological dissimilarities between mammals' brains and the brains of other animals are massive: some of the structures that support consciousness in mammals are absent.
Conclusion 4.32: Similarity arguments based on behaviour cannot establish the occurrence of conscious feelings in animals when there is conflicting neurological evidence.
Before suggesting a solution to this evidential conundrum, I shall briefly summarise the neural requirements for consciousness.