On the Improvement of Understanding, by Benedict de Spinoza, [1883], at sacred-texts.com
[77] (1) Thus far we have treated of the false idea. We have now to investigate the doubtful idea—that is, to inquire what can cause us to doubt, and how doubt may be removed. (2) I speak of real doubt existing in the mind, not of such doubt as we see exemplified when a man says that he doubts, though his mind does not really hesitate. (77:3) The cure of the latter does not fall within the province of method, it belongs rather to inquiries concerning obstinacy and its cure.
[78] (1) Real doubt is never produced in the mind by the thing doubted of. (2) In other words, if there were only one idea in the mind, whether that idea were true or false, there would be no doubt or certainty present, only a certain sensation. (3) For an idea is in itself nothing else than a certain sensation. (4) But doubt will arise through another idea, not clear and distinct enough for us to be able to draw any certain conclusions with regard to the matter under consideration; that is, the idea which causes us to doubt is not clear and distinct. (5) To take an example. (78:6) Supposing that a man has never reflected, taught by experience or by any other means, that our senses sometimes deceive us, he will never doubt whether the sun be greater or less than it appears. (7) Thus rustics are generally astonished when they hear that the sun is much larger than the earth. (8) But from reflection on the deceitfulness of the senses [78a] doubt arises, and if, after doubting, we acquire a true knowledge of the senses, and how things at a distance are represented through their instrumentality, doubt is again removed.
[79] (1) Hence we cannot cast doubt on true ideas by the supposition that there is a deceitful Deity, who leads us astray even in what is most certain. (2) We can only hold such an hypothesis so long as we have no clear and distinct idea—in other words, until we reflect the knowledge which we have of the first principle of all things, and find that which teaches us that God is not a deceiver, and until we know this with the same certainty as we know from reflecting on the are equal to two right angles. (3) But if we have a knowledge of God equal to that which we have of a triangle, all doubt is removed. (79:4) In the same way as we can arrive at the said knowledge of a triangle, though not absolutely sure that there is not some arch-deceiver leading us astray, so can we come to a like knowledge of God under the like condition, and when we have attained to it, it is sufficient, as I said before, to remove every doubt which we can possess concerning clear and distinct ideas.
[80] (1) Thus, if a man proceeded with our investigations in due order, inquiring first into those things which should first be inquired into, never passing over a link in the chain of association, and with knowledge how to define his questions before seeking to answer them, he will never have any ideas save such as are very certain, or, in other words, clear and distinct; for doubt is only a suspension of the spirit concerning some affirmation or negation which it would pronounce upon unhesitatingly if it were not in ignorance of something, without which the knowledge of the matter in hand must needs be imperfect. (2) We may, therefore, conclude that doubt always proceeds from want of due order in investigation.