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.
On Davidson s view, semantic opacity is related to the holistic character of thought. The beliefs we have are determined in part by their causes and in part by the space of other beliefs in which they are embedded. Because Spot lacks this space of other beliefs, he lacks a semantically opaque belief about the tree. An argument of this form seems to conflate an epistemic point about evidence we might have for thoughts with a claim about the actual possession of those thoughts. Let us bracket this worry and consider a distinct strand of reasoning.