be retained. For, inasmuch as we desire to form an idea of man as a type of human nature which we may hold in view, it will be useful for us to retain the terms in question, in the sense I have indicated.Leibniz adhered to the doctrine as well, and employed it as part of his theodical argument that the actual world is the best of all possible worlds. Late modern religion The doctrine is also held by the Baháʼí Faith. ʻAbdu'l-Bahá stated to a French Baháʼí woman: Criticism Bertrand Russell criticized the doctrine in his essay The Elements of Ethics: Sorrow Pain and sorrow, mentioned by Russell in the quote above, are popular alleged counterexamples. The Stanford Encyclopedia of Philosophy article on "The Concept of Evil", written by philosopher Todd Calder, also says that "it seems that we cannot equate the evil of pain with the privation of pleasure or some other feeling. Pain is a distinct phenomenological experience which is positively bad and not merely not good." Thomas Aquinas, a proponent of the privation theory, argued against this opinion in his Summa Theologiae:[...] supposing the presence of something saddening or painful, it is a sign of goodness if a man is in sorrow or pain on account of this present evil. For if he were not to be in sorrow or pain, this could only be either because he feels it not, or because he does not reckon it as something unbecoming, both of which are manifest evils. Consequently it is a condition of goodness, that, supposing an evil to be present, sorrow or pain should ensue. Opposition between appearances Immanuel Kant believed that, while the doctrine is true of concepts of the understanding, it is nevertheless false about the world as it appears to the senses. In a remark to the section of the Critique of Pure Reason entitled the "Amphiboly of the Conceptions of Reflection", he criticized the Leibnizian school of philosophy for not acknowledging this possibility of merely phenomenal opposition:The principle: "Realities (as simple affirmations) never logically contradict each other," is a proposition perfectly true respecting the relation of conceptions, but, whether as regards nature, or things in themselves (of which we have not the slightest conception), is without any the least meaning. For real opposition, in which A − B is = 0, exists everywhere, an opposition, that is, in which one reality united