Why did W.V.O. Quine think intentionality was hard to reduce to physical facts?

In the context of the philosophy of mind, W.V.O. Quine’s skepticism regarding the reduction of intentionality to physical facts stems from his broader views on language and meaning. Quine argued that there is no "fact of the matter" regarding the specific content of our thoughts or the meanings of our words.12

This position is rooted in his thesis of the indeterminacy of translation. Quine posited that one could construct multiple, non-equivalent "translation manuals" that are equally compatible with all available empirical evidence. Because there is no objective, determinate meaning to be discovered, it becomes impossible to identify which specific physical facts about a brain correspond to a particular thought.123

For example, Quine questioned what physical facts could definitively determine whether a person is thinking about a "rabbit," "rabbithood," or "undetached rabbit parts". Because intentional states are "referentially opaque," they resist the kind of straightforward physical reduction that applies to non-mental phenomena, where microphysical facts "upwardly necessitate" macrophysical outcomes. Consequently, Quine viewed mentalistic notions like "meaning" as scientifically suspect.3412