But in many cases, when we are dealing with an object about which nothing can be done by us, and in regard to which our judgment is therefore purely theoretical, we can conceive and picture to ourselves an attitude for which we regard ourselves as having sufficient grounds, while yet there is no existing means of arriving at certainty in the matter. Thus even in purely theoretical judgments there is an analogon of practical judgments, to the mental entertaining of which the term 'belief' is appropriate, and which we may entitle doctrinal belief. I should be ready to stake my all on the contention--were it possible by means of any experience to settle the question--that at least one of the planets which we see is inhabited. Hence I say that it is not merely opinion, but a strong belief, on the correctness of which I should be prepared to run great risks, that other worlds are inhabited.
Immanuel Kant, Critique of Pure Reason A825/B853 (Kemp Smith, tr.)