There are three kinds of certainty. First, absolute certainty. This is the certainty which lies in necessary and eternal principles e. g. 2 x 2=4; the existence of space; every body must be in space; every phenomenon must have a cause; the being of God.
Logical certainty, that is, the connexion between premises and conclusion, is likewise absolute.
Secondly. Physical certainty. This is the certainty which lies in the connexion between physical causes and their phenomena: e. g. gravitation, heat, chemical affinities in general, mechanical forces.
The reason conceives of these causes as inherently active and uniform; and hence, wherever a physical cause exists, we expect its proper phenomena.
Now we do not call the operation of these causes absolutely certain, because they depend ultimately upon will,—the will of God; and we can conceive that the same will which ordained them, can change, suspend, or even annihilate them: they have no intrinsic necessity, still, as causes given in time and space, we conceive of them generally as immutable. If in any case they be changed, or suspended, we are compelled to recognise the presence of that will which ordained them. Such change or suspension we call a miracle; that is, a surprise,—a wonder, because it is unlooked for.
When, therefore, we affirm any thing to be physically certain, we mean that it is certain in the immutability of a cause acting in time and space, and under a necessity relatively to the divine will; but still not absolutely certain, because there is a possibility of a miracle. But when we affirm any thing to be absolutely certain, we mean that it is certain as comprehended in a principle which is unalterable in its very nature, and is therefore independent of will.
Thirdly. Moral certainty, is the certainty which lies between the connexion of motive and will. By will we mean a self-conscious and intelligent cause, or a cause in unity with intelligence. It is also, in the fullest sense, a cause per se; that is, it contains within itself proper efficiency, and determines its own direction. By motives we mean the reasons according to which the will acts. In general, all activity proceeds according to rules, or laws, or reasons; for they have the same meaning: but in mere material masses, the rule is not contemplated by the acting force,—it is contemplated only by the intelligence which ordained and conditioned the force. In spirit, on the contrary, the activity which we call will is self-conscious, and is connected with a perception of the reasons, or ends, or motives of action. These motives or ends of action are of two kinds. First, those found in the ideas of the practical reason, which decides what is fit and right. These are reasons of supreme authority. Secondly, those found in the understanding and sensitivity: e. g. the immediately useful and expedient, and the gratification of passion. These are right only when subordinate to the first.
Now these reasons and motives are a light to the will, and serve to direct its activities; and the human conscience, which is but the reason, has drawn up for the will explicit rules, suited to all circumstances and relations, which are called ethics, or the rules.
These rules the will is not compelled or necessitated to obey. In every volition it is conscious of a power to do or not to do; but yet, as the will forms a unity with the intelligence, we take for granted that it will obey them, unless grounds for an opposite conclusion are apparent. But the only probable ground for a disobedience of these rules lies in a state of sinfulness,—a corruption of the sensitivity, or a disposition to violate the harmony and fitness of the spiritual constitution. Hence moral certainty can exist only where the harmony of the spiritual being is preserved. For example: God and good angels. In God moral certainty is infinite. His dispositions are infinitely pure, and his will freely determines to do right; it is not compelled or necessitated, for then his infinite meritoriousness would cease. Moral certainty is not absolute, because will being a power to do or not to do, there is always a possibility, although there may be no probability, nay an infinite improbability, that the will may disobey the laws of the reason.
In the case of angels and good men, the moral certainty is such as to be attended with no apprehension of a dereliction. With respect to such men as Joseph, Daniel, Paul, Howard, and Washington, we can calculate with a very high and satisfactory moral certainty, of the manner in which they will act in any given circumstances involving the influence of motives. We know they will obey truth, justice, and mercy,—that is, the first class of motives; and the second only so far as they are authorized by the first. If the first class of motives are forsaken, then human conduct can be calculated only according to the influence of the second class.