Full Idea
We are so far from knowing what figure, size or motion of parts produce a yellow colour, sweet taste, or sharp sound, that we cannot conceive how any size, figure or motion can produce in us the colour, taste or sound. There is no conceivable connection.
Gist of Idea
We can't begin to conceive what would produce some particular experience within our minds
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.03.13)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.292
A Reaction
There is a good case for naming Locke as the first mysterian, and he puts his finger here on what I think is the weirdest puzzle of the mind - why THAT experience for THAT stimulus. In the 21st century we should not give up so easily.