By a property (_proprium_, ἴδιον) we understand a quality which, though not part of the essence of a thing, necessarily flows from that essence by some sort of causation and is consequently found in all individuals of the same species.(1155) A property, as such, is opposed to an accident (_accidens_, συμβεβηκός), which is neither part of, nor necessarily attached to, the essence, but may or may not be present in the individual. Thus the ability to laugh is a property of human nature, whereas the color of the skin is an accident. How do the properties of grace differ from its formal effects, and from its supernatural concomitants? The formal effects of grace, as we have seen, are the elements constituting its nature, the properties are determinations necessarily flowing from that nature, while the supernatural concomitants are free gifts superadded by God. According to the Protestant theory, justification is absolutely certain, equal in all men, and incapable of being lost. The Catholic Church, on the contrary, teaches that justification is (1) uncertain, (2) unequal, and (3) amissible. We will explain this teaching in three theses. *Thesis I: No man knows with certainty of faith whether he is justified or not.* This proposition is _de fide_. Proof. The Tridentine Council rejected the “fiduciary faith”(1156) of Luther as “an empty heretical confidence,”(1157) and in three distinct canons denied the properties attributed to faith by the early Protestant dogmaticians.(1158) a) Holy Scripture again and again warns us that we can never be sure of our salvation. St. Paul, though himself “a vessel of election,” freely admits: “I am not conscious to myself of any thing, yet I am not hereby justified; but he that judgeth me is the Lord,”(1159) and declares: “I chastise my body and bring it into subjection, lest perhaps, when I have preached to others, I myself should become a castaway.”(1160) He exhorts the faithful to work out their salvation “with fear and trembling.”(1161) b) The Fathers also teach the uncertainty of justification in the individual, and attribute it to the fact that, while we know that God pardons penitent sinners, no man can be entirely certain that he has complied with all the conditions necessary for justification.