If you are used to arguing for animal rights you've certainly heard this kind of protest more than once:
"How do you know that the chickens feel pain? Have they told you?"
You certainly haven't related to this seriously. It's clear that this is generally a put-down. However, there are skeptics with a more refined approach.
"Pain? Perhaps. But claiming that they are miserable just because they are caged, or because they are denied conditions which they have never known - this is anthropomorphism!"
It's much more difficult to ignore this kind of argument. There are also those who add, with a victorious look,
"They can't communicate with us by language so therefore their sufferings and needs are just speculation; observation of their behavior is not enough for us to get an idea of what is going on in their heads."
This argument appears to be almost correct! Therefore it is worth looking into this matter more deeply, under the limitations of a short essay. Under what conditions can we say anything meaningful about 'what goes on in their heads'? For this purpose we need to start with much more general questions, then we'll return to the chickens.
My friend the robot
When I try to gauge 'what is going on in another person's head' I have to guess. We each think our own thoughts. We can't see the thoughts of another, hear or feel them. We can only see, for example, how his pupils dilate or how his hand shakes. We can also measure the level of adrenaline in his blood. If we conclude from this that he is angry – it is still nothing but a guess. Perhaps we have erred in the interpretation of the signs and in fact he feels completely different? Perhaps he is actually a robot lacking awareness, programmed to dilate pupils, quiver limbs, secrete adrenaline and all the other signs that we use to interpret as symptoms of thoughts?
Language? What's the big deal!
Obviously, to know what my friend is thinking, I will just ask him. However, his replies don't prove what he is thinking, or that he is thinking at all. We still haven't solved the problem of the robot – perhaps he's programmed to speak? What if he is a normal person but he speaks a language I don't understand – what use can I make of his talking then? Or perhaps he is just lying? In these cases we derive our conclusions about 'what goes on in his head' from observing his behavior and the environmental conditions around him. And we do it in a way which is similar to the way we study animals which don't have a language.
A practical thought on the thought of another
Should this be the case, we cannot know with certainty what goes on in the head of another person. Even so we behave as if we know with certainty what others are thinking. We all have exhaustive circumstantial data about the thoughts of others. Given this, there's no reason why we can't rely on similar circumstantial evidence for the purpose of gauging the thoughts and feelings of animals of other species. Of course the information we have concerning them is much more restricted in comparison with all that is known to us about normal adult human beings, yet we still have a large body of knowledge, which enables us to make a number of assumptions with a reasonable degree of certainty; these assumptions are no less 'certain' than those we may have concerning the awareness of human beings. This information is sufficient only if we use all the following rules and not just one or two of them.
Rule One: Testing the body
We have a great deal of information about the structure and function of the nervous system of different animals. Many species, particularly vertebrates (ourselves included) have a nervous system which resembles that of ours – from the sensory cells at the tips of our limbs all the way to our brains. Their nervous systems respond to the environment in a manner very similar to the responses of human beings, on the levels of the function of specific chemicals in the body, of cells, or of whole networks. It's reasonable to assume that given the similarity between the physical phenomena in our nervous systems and that of other species, there is also a similarity in states of mind connected to these phenomena. (This is not an invitation for experiments on animals! It is absurd to perform experiments on animals in order to prove eventually that such experiments are morally wrong!)
Rule Two: Observing behavior
Since our behavior expresses our state of mind and our thoughts, it is reasonable to assume that specific behaviors of other animals do express feelings and thoughts as well. Pain, fear, anxiety and enjoyment are expressed through behavior (flight, shaking, respiration, distress cries, care of an injured limb, etc. – the list is endless). We can also learn from animals' behavior about their complex preferences (for example, when an animal has several options she chooses one of them consistently). In the case of complex social animals the signals exchanged between them provide additional evidences as to 'what goes on in their heads'.
Rule Three: Reflecting on evolution
The great similarity between us and other species is not accidental. We can assume that we have developed from early common ancestors, and therefore the function of our own nervous systems cannot be arbitrarily different from the function of the nervous systems of other animals. The evolutionary framework demands even more than this: any characteristic that we identify in any animal has developed through natural selection, that is to say, it must have an advantage for survival. For example, mental complexity is an advantage for an animal that lives many years and exists in a society, but not for creatures that hatch in adult form and live for only one day.
What are the implications of all this?
First of all, we see that skeptical statements of the sort "we don't know what goes on in the heads of animals" have no practical relevance, just as similar doubts concerning other people's mind is irrelevant for practical purposes. Secondly, in order to know something about 'what goes on in the head' of any animal it's not enough to rely on scanty information of a limited sort. Some behavioral data, some physiological data – these are not enough in order to seriously appreciate "what goes on in their head." If someone brings forth "proof" that chickens don't suffer in the cages of the egg industry, or that they are not capable of feeling suffering, we can see straight away that this "proof" is no more than limited data that does not follow the requirement to combine the three tests discussed here. In order to protect the rights of animals there is, of course, a need to know a great deal of data of all types. Talking about severe injury of vertebrates, the important data clearly exists.