It was, as Jay points out, Dr. Sam. Here's the passage from Boswell:
After we came out of the church, we stood talking for some time
together of Bishop Berkeley's ingenious sophistry to prove the
non-existence of matter, and that every thing in the universe
is merely ideal. I observed, that though we are satisfied his
doctrine is not true, it is impossible to refute it. I never
shall forget the alacrity with which Johnson answered, striking
his foot with mighty force against a large stone, till he
rebounded from it, "I refute it _thus_." (6 August 1763).
And then there's Richard Wilbur:
Kick at the rock, Sam Johnson, break your bones:
But cloudy, cloudy is the stuff of stones.
-- Russ
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