Full Idea
The fact that we often see two things joined together does not license the inference that they are one and the same.
Gist of Idea
Two things being joined together doesn't prove they are the same
Source
René Descartes (Reply to Sixth Objections [1641], 444)
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.115
A Reaction
Correct. The problem comes when they are never ever apart, and you begin to suspect that they are conjoined in all possible worlds. Why might this be so? It can only be identity or a causal link.