Thus, if a cause be defined, 'that which produces any thing', it is easy to observe, that 'producing' is synonymous to 'causing'. In like manner, if a cause be defined, 'that by which any thing exists'; this is liable to the same objection. For what is meant by these words, 'by which'? Had it been said, that a cause is that after which any thing constantly exists; we should have understood the terms. For this is, indeed, all we know of the matter. And this constancy forms the very essence of necessity, nor have we any other idea of it.