Every schoolboy knows it.

The phrase is a rhetorical flourish meaning “this is common knowledge,” used to shame or disarm an opponent by implying the point is so elementary that even a child would know it. In polemical or sermonic prose (a mode strongly associated with Jeremy Taylor), such a move can function as an appeal to shared cultural literacy and to the authority of received learning. However, without a verified location in Taylor’s writings, it is difficult to say what specific “it” refers to or what argumentative purpose it served in its original setting.