Human language brings clarity and understanding to human thoughts and beliefs. In fact, many have argued that without language, humans have no capacity for thought or belief. Descartes expresses a firm conviction that language is necessary for any thought:
There has never been an animal so perfect as to use a sign to make other animals understand something which bore no relation to its passions; and there is no human being so imperfect as not to do so. . . . The reason animals do not speak as we do is not that they lack the organs but that they have no thoughts. It cannot be said that they speak to each other but we cannot understand them; for since dogs and some other animals express their passions to us, they would express their thoughts also if they had them. (CSMK 575)
While the idea that language is necessary for the emergence of belief has been accepted for centuries, philosophers and others have begun to use the term “belief” more permissively, making the assertion much less obvious. While to say a cow had beliefs may have once implied the cow ascribed to some creed or doctrine, the claim has a much more mundane connotation in contemporary philosophy. For example, using the language of belief/desire psychology, we might say that a group of cows and humans gathering under a cover after hearing a thunderclap share a common belief that it is about to rain.
We will also say they desire to stay out of the storm. Cows do not need the ability to express their beliefs to want to avoid a storm that appears to be imminent. In this case, it is easy to describe the cow’s behavior using the language of belief/desire psychology, but it is also easy to imagine that the humans under the cover are in a far different position than the cows; they understand their position, have plans and fears for the future, and have a sense of what it is right and wrong to do. We want to say the humans are conscious, and the cows are not. We know the humans are conscious because we assume them to be more or less like us, and we are conscious. Language expresses our thoughts and beliefs, and we assume that other humans use language and experience consciousness as we do.
Language does more than provide evidence of consciousness, though; it is the structure of consciousness. A sophisticated study of human language and behavior should produce a powerful and accurate psychological theory. If language sets humans apart from machines and animals, then language is quite likely the feature of human consciousness that produces moral agency and responsibility. If animals and machines are not capable of beliefs and thoughts, then humans are the only known creatures to have any concept of moral responsibility. However, if consciousness is not unique to humans, or if language is not the stuff that makes consciousness, then we may not be able to construct an adequate description of beliefs and desires, much less moral agency.