A certain je ne sais quoi

J speaks of a 'certain je ne sais quoi' that Descartes has that Locke lacks. I agree.

I'm worried about 'certain', though.

It seems to me that if you say, in French, 'je ne sais quoi', you mean that you don't know (something). So how can you be certain about it?

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J would suggest that 'certain' has two senses or 'uses'. In the first sense, 'certain' means something the Pope is -- as when we say he is ineffable. He is ineffably certain (about things) -- even if wrong.

The second use is when we say,

"A certain prostitute was walking the dark alley".

We don't mean anything about the epistemic state of the prostitute or the reporter. In that case, 'certain' means (Ex). The logical form being:

(Ex)Px & W(da, x).

where "P" stands for 'prostitute' and 'da' for 'dark alley' (and 'walk' for 'w').