Venerable Fulton Sheen (and St. Pius X, G. K. Chesterton, C. S. Lewis, Pius XII, etc., etc., etc.) identified the greatest danger to Catholic doctrine today (and, oddly enough, to all religion, organized or otherwise) as the rejection of the first principle of reason. This first principle of reason is that a thing cannot both "be" and "not be."
This is more positively stated as that which is true, is as true, and is true in the same way as everything else that is true. In other words, when a thing is a thing, it is that thing and no other. The principle of contradiction and the principle of identity are really just two sides of the same coin, two ways of saying the same thing.
Many people will claim you're diverting the discussion from the real issue (i.e., whatever [stuff] they're pushing) if you raise this, but once you've abandoned this principle (which, as Sheen points out, is "mental suicide"), nothing you say has any relevance. This was the subject of Sheen's God and Intelligence (1925), Chesterton's St. Thomas Aquinas: The Dumb Ox (1933), and Pius XII's Humani Generis (1950).
This modern heresy is not so much an attack on any specific doctrine or truth of the natural law as it is an attack on reason itself, which allows you to make things mean anything you want, e.g., John Maynard Keynes's claim that the State has the right to "re-edit the dictionary." Consequently, truth ceases to mean anything, and "right" is determined by whoever has the biggest club or the most money.