Knocking his sconce
How did Aristotle come to his a priori, intuitive conviction that material bodies actually exist? "How? By knocking his sconce against them, sure." Stephen here is fictively allying Aristotle with the 18th century man of letters Samuel Johnson, who had no patience for Bishop Berkeley's "ingenious sophistry" (in the words of James Boswell) "to prove the non-existence of matter, and that everything in the universe is merely ideal." Asked to refute Berkeley's position, Johnson did so physically rather than argumentatively.
Boswell writes, "I shall never 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.'" Adding his own thought experiment to Johnson's stone-kicking and Aristotle's noggin-knocking, Stephen thinks, "If you can put your five fingers through it it is a gate, if not a door. Shut your eyes and see." The sense of touch can confirm things that vision may leave ambiguous.
The allusion to Boswell's Life of Johnson was first noticed by Robert M. Adams, in Surface and Symbol: The Consistency of James Joyce's Ulysses (Oxford, 1962), 134.