It s important to distinguish the hypothesis that language and thought are interdependent from reductionist hypotheses. We identify thoughts, distinguish between them, describe them for what they are, only as they can be located within a dense network of related beliefs. If we really can intelligibly ascribe single beliefs to a dog, we must be able to imagine how we would decide whether the dog has many other beliefs of the kind necessary for making sense of the first. It seems to me that no matter where we start, we very soon come to beliefs such that we have no idea how to tell whether a dog has them, and yet such that, without them, our confident first attribution looks shaky. There is no fixed list of things someone with the concept of a tree must believe, but without many general beliefs there would be no reason to identify a belief as a belief about a tree.
Mute creatures have no thoughts.
From what has been said about the dependence of beliefs on other beliefs it is clear that a very complex pattern of behavior must be observed to justify the attribution of a single thought. Or, more accurately, there has to be good reason to believe there is such a complex pattern of behavior. And unless there is actually such a complex pattern of behavior, there is no thought. On the face of it this is a terrible argument. Thoughts evidently exhibit a fine-grained character.