CERTAINTY. - From cernere, to sift, to discern, certainty is, in the proper sense, the mental state of firm adherence to a proposition. St. Thomas: «certitudo proprie dicitur firmitas adhaesionis virtutis cognoscitivae ad suum cognoscibile»; «certitudo nihil aliud est quam determinatio intellectus ad unum» (In III Sent., dist. 26, II, 4; ibid., dist. 23, II, 2, sol. III).
Certainty admits of various distinctions, especially in relation to the diversity of the objects of knowledge and of the conditions that ground it.
Common since Leibniz (Now. ess., IV, 6, 13) is the division of certainty into metaphysical, physical and moral, which Balmes calls respectively absolute, natural and ordinary. Metaphysical certainty is that founded on the absolute necessity of the essential laws of reality (the whole is greater than the part); physical certainty that founded on the stability and constancy of natural laws (tomorrow the sun will rise); moral certainty that founded on the ordinary validity of the practical laws of life (war makes men worse); it is evident, however, that in this last case one cannot speak of certainty in the proper sense, since there is lacking an objectively and universally valid foundation sufficient to determine thought with assurance. The expression "moral certainty" has, moreover, several other meanings. Sometimes it is taken as a synonym for maximum probability; often it designates practical certainty regarding the morality of a determinate action, a certainty not necessarily bound, as in the case of a doubtful law, to theoretical evidence. With the expression "ethical certainty" it is today frequently intended to affirm the particular condition that the perception of moral values possesses, attributed not so much to an evidence of a conceptual order as rather to intuition or to moral feeling.
As appears from its notion, certainty is a subjective modality of knowledge; nevertheless one also customarily speaks of objective certainty as opposed to subjective, in order to distinguish legitimate certainty founded on the true from the purely psychological state of assured persuasion that sometimes accompanies error. Finally, one distinguishes spontaneous certainty, proper to reason in its natural and pre-scientific exercise, from reflected certainty, the fruit of an explicit becoming-aware on the part of thought of its own affirmations and of the motives that ground them. In practice, however, this distinction is not as clear-cut as it may seem, since an initial reflection is always present wherever the impossibility of the opposite is explicitly known, as is the frequent case of spontaneous certainty.
The reasons that determine certainty in the spirit, understood in the proper sense indicated above, are and must be objective, that is, inherent in the object of the judgment: thought can halt with absolute assurance in an object of knowledge only insofar as this object manifests itself to it in the light of its truth, that is, of its interior necessity.
A state of purely subjective persuasion can certainly arise from various and manifold causes, both internal and external, not excluding desire, passion, and any psychological consonance whatsoever; but full and conscious certainty is always, in the last analysis, determined by the evidence of being that opens itself in its intelligibility to thought: concrete evidence of intuition in relation to facts and situations of consciousness, and logical evidence of the connection of concepts with regard to judgments of an ideal order. With this, while rejecting the fideistic and voluntaristic position (Reid, Jacobi, Jouffroy, Goblet, etc.) that relegates the determinants of certainty to the field of instinct and will, one does not exclude that it may often mature upon a complex of psychological dispositions and coefficients in which, besides the intellect, the will and the heart have their part. This holds especially — not to speak of aesthetic certainty, essentially alogical and therefore of feeling — for the truths of faith (v.), where only an extrinsic evidence is possible, based on the certainty of revelation and of divine infallibility. Today, however, outside of scholastic terminology, the persuasion of faith is for the most part designated simply by the term faith or belief (v.), reserving the name of certainty for the persuasion of an intellective order derived from the intelligence.
At various times in the history of thought the question has been raised whether man can truly attain certainty. Evidently it is not a question of spontaneous certainty, a universal fact of immediate experience, but of reflected certainty, absolutely valid in the philosophical sphere and as a matter of principle. The sceptical claim (v. SCETTICISMO) takes strength from the perennial and, it would seem, insuperable crisis of truth in the human mind. But the answer in the positive sense admits of no doubt, once thought but re-enters itself and becomes conscious of the absolute values of which it finds itself in possession: in the varied and rich experience of itself, thought holds in its hand the firmest certainties, superior and refractory to every doubt. In these first certainties of fact grasped in self-consciousness and which are immediately translated into certainties of principle, there lies the surest foundation for an affirmation in a general sense of certainty in the field of philosophy and of life (v. CONOSCENZA; CRITERIO; EVIDENZA).