By Hume's fork, sheer conceptual derivations (ostensibly, logic and mathematics), being analytic, are necessary and "a priori", whereas assertions of "real existence" and traits, being synthetic, are contingent and "a posteriori". Hume's own, simpler, distinction concerned the problem of induction—that no amount of examination of cases will logically entail the conformity of unexamined cases—and supported Hume's aim to position humanism on par with empirical science while combatting allegedly rampant "sophistry and illusion" by philosophers and religionists. Being a Transcendental Idealist, Kant asserted both the hope of a true metaphysics, and a literal view of Newton's law of universal gravitation by defying Hume's fork to declare the "synthetic a priori." In the 1930s, the logical empiricists staked Hume's fork. Yet in the 1950s, W. V. O Quine undermined its analytic/synthetic distinction. And in the 1970s, Saul Kripke established the necessary "a posteriori". Still, Hume's fork is a useful starting point to anchor philosophical scrutiny.