 [O]aths are taken for the purpose of confirmation. Now speculative propositions receive confirmation from reason, which proceeds from principles known naturally and infallibly true. But particular contingent facts regarding man cannot be confirmed by a necessary reason, wherefore propositions regarding such things are wont to be confirmed by witnesses. Now a human witness does not suffice to confirm such matters for two reasons. First, on account of man’s lack of truth, for many give way to lying, according to Ps. 16:10, “Their mouth hath spoken lies [Vulg.: ‘proudly’]”. Secondly, on account of [his] lack of knowledge, since he can know neither the future, nor secret thoughts, nor distant things: and yet men speak about such things, and our everyday life requires that we should have some certitude about them. Hence the need to have recourse to a Divine witness, for neither can God lie, nor is anything hidden from Him. Now to call God to witness is named jurare (to swear) because it is established as though it were a principle of law (jure) that what a man asserts under the invocation of God as His witness should be accepted as true. Now sometimes God is called to witness when we assert present or past events, and this is termed a “declaratory oath”; while sometimes God is called to witness in confirmation of something future, and this is termed a “promissory oath”. But oaths are not employed in order to substantiate necessary matters, and such as come under the investigation of reason; for it would seem absurd in a scientific discussion to wish to prove one’s point by an oath. (Summa II-II q.89-Art. i)