165. The various sources of certainty, examined in the preceding chapter, furnish us motives of certainty, i.e., reasons which move our intellect to elicit firm undoubting judgments. But these several sources do not give certainty except when properly applied to their proper objects; e.g., our senses are not reliable except under the proper conditions. Hence, to have philosophic certainty in any given case, we must examine whether in that case all the necessary conditions have been complied with, and whether no reason remains to entertain any further doubt. For this purpose we need a rule or test by which to judge our very judgments; to ascertain beyond the possibility of error that they are conformable to the objective trutb. This rule to judge by is called a criterion (krinô, I judge) of certainty.
166. We maintain that the ultimate and universal criterion of certainty is the evidence of the objective truth. By calling it ultimate, or last, we mean that, when this criterion is applied, it leaves no room for further inquiry concerning the existence of certainty; the ultimate criterion answers the last question that we can or need ask in examining the reliability of our knowledge. For instance, if I question myself how I know that bodies exist, I answer that I see and feel them, that by my senses I perceive their existence, and I cannot perceive that which does not exist as an object of perception; in other words, their existence is made evident to me. If asked why I am certain that the Declaration of Independence occurred in the United States, I answer that I have learned it from reliable witnesses. And why do I believe these witnesses? Because my reason convinces me that their testimony is reliable. But why do I rely on my reason? Because it gives me evident conclusions from certain premises. I can question no further, because I can wish for nothing more evident than evidence.
By calling evidence the universal criterion of certainty, we mean that evidence is the crucial test in all cases of natural certainty; for it is with natural certainty alone, not with supernatural Faith, that Philosophy is concerned.
167. What, then, is evidence? It is important to understand it well, since all certainty is ultimately to be tested by this criterion. As stated above (No. 84), in the analysis of certainty we find that the firm adhesion of our mind to a ruth, excluding all fear of error, is the subjective element of certainty; and the manifestation of the truth to the mind producing this firm adhesion, is the objective element. Now, such manifestation is the evidence of that truth. Evidence is to the mind what the visibility of a body is to the eye. That I may see a body, 1. It must exist; 2. It must give forth, or at least reflect, rays of light; 3. By that light it must impress itself on my eye. So, likewise, that a truth may be evident to me, 1. It must exist; 2. It must shine forth by its intelligibility, as all truth does, for ontological truth is the intelligibility of a thing; 3. Its light or intelligibility must be so presented as to force itself upon my intellect, making me see that the thing is so and must be so, cannot be otherwise. Hence a usual and correct definition of evidence is "such a manifestation of a truth as makes us see that the thing is so and cannot be otherwise," or, more briefly, "the manifest necessity of a truth." We do not mean here that the objective truth is absolutely necessary, but only that, I see it, it must be, else I could not see it; the truth is hypothetically necessary.
168. Before Descartes' time, the fact that evidence is the ultimate criterion of certainty was scarcely disputed; but this writer has so confused the question of certainty that many modern philosophers have assigned and defended false criteria. Descartes himself considers clear ideas as the great test or principle of certainty; while Reid, and the Scottish School generally, rely ultimately upon what they call common sense by which they mean a blind instinct to consider a thing as true. But they should prove that such ideas or such an instinct is necessarily a pledge of the objective truth. In fact, these criteria are all internal; now, no merely internal test can settle the question whether the external things exist, since it is not necessarily connected with the objective truth.
Others look for the criterion in a merely external rule. Thus, De Lamennais, indignant that human reason had been adored in France during the Reign of Terror, strove to discredit reason and to show that we cannot trust our reason but must test its reliability by comparing its judgments with the common consent of men. But how can we know that men are agreed upon any point, unless we can rely on our senses and our reason to ascertain whether men exist and what they say? No merely external test can be ultimate; for we need a further criterion to judge of its existence and its reliability.
169. Thesis XV. The evidence of the objective truth is the ultimate and universal criterion of certainty.
Proof. It is such if it fulfils the following conditions: 1. To be a reliable test of truth, the criterion must be inseparable from the truth, so that it cannot exist without the truth. 2. To be ultimate, it must leave no doubt to be removed by a farther test. 3. To be universal it must be applicable to every motive of natural certainty. Now, the evidence of the objective truth, and it alone, fulfils all these conditions: 1. It cannot exist without the truth, since it is the intelligibility of the truth itself made manifest to us. 2. It leaves no doubt to be removed by a further test, since it enables the mind to see the necessity of the truth manifested to it. 3. It is universal, for in no case have we real certainty unless we see that the truth is so and cannot be otherwise; but this supposes the evidence of the objective truth, and is nothing else than the perception of that evidence. Therefore this evidence is the ultimate and universal criterion of certainty.
170. Objections: 1. We cannot be certain of anything unless we know that others agree with us. Answer. We deny this. In fact, we could not know that others agree with us if our own faculties were not reliable, capable of seeing the evidence of that agreement.
2. We cannot know that we are not insane except by ascertaining that others agree with us. Answer. This, too, we deny. Besides, even an insane man cannot err when he has evidence; but he has not evidence in the matters wherein he is insane; for evidence is a manifestation of the truth. It must, besides, be remembered that we claim certainty for man in his normal state, not for crazy, drunken, or sleeping men; and the very reason why these cannot be certain is be cause they cannot reflect sufficiently to examine their judgments: they imagine that things are so, but they cannot see that things cannot be otherwise.
3. Evidence is only in our minds. Answer. True evidence is the light of objective truth perceived by our minds; that which is not cannot be perceived.
4. We cannot be certain that we have evidence. Answer. We can, as our consciousness testifies.
5. Every man is fallible. Answer. Not about matters that are evident.
6. We have no infallible knowledge except through Revelation. Answer. We have; and we could not rationally trust a Revelation if we had no evidence that it was made: those who attack the reliability of our reason thereby attack the foundation of Faith.
7. God is the ultimate motive of certainty. Answer. He is the first being existing and knowing, but not the first being known to me: His existence is first ontologically, not logically, with regard to me.
8. Consciousness is the ultimate criterion of certainty; for it answers the last question asked about the motives of certainty. Answer. We trust our consciousness because it is evidently reliable, thus evidence is the ultimate criterion.
9. Evidence itself requires attention and examination. Answer. As motives of assent, no; as necessary conditions for the existence of subjective evidence, yes.
10. Evidence does not reach all kinds of truth; for instance, we have no evidence of what we learn from witnesses. Answer. We have no intrinsic evidence of it, but extrinsic, i.e., we have evidence that the witnesses could not deceive us.
11. An evident conclusion may be false. Answer. Not if the whole reasoning is evident, premises and sequence.
12. It is the part of Protestantism to make one's own judgment the criterion of all certainty. Answer. Protestantism errs in making private judgment the criterion of supernatural certainty.
13. Many truths are certain, but not evident. Answer. Of natural truths all that are certain are either intrinsically or extrinsically, directly or indirectly, evident to man's natural powers.