For the better understanding of the subject now before us, we will recall the true state of the question. We do not seek a first principle, which of itself alone illumines or produces all truths; but we seek a truth which shall be the indispensable condition of all knowledge; for this reason, we do not call it an origin, but a resting-point. The edifice does not originate in the foundation, but rests upon it. We must consider the principle sought for as a foundation, just as in the preceding chapters, we treated of discovering a seed. These two images _seed_ and _foundation_, perfectly express our ideas, and exactly trace the limits of the two questions. 144. Is there a resting-point of all science, and of all knowledge, scientific or not scientific? If there is, what is it? Are there many, or only one? Evidently there must be a resting-point. If asked the reason of an assent, we must at last come to a fact or a proposition, beyond which we cannot go; for we cannot admit the process _ad infinitum_. This is the point where we must of necessity stop, and consequently the resting-point of certainty. 145. Starting with a given assent, we may, perhaps, arrive at different principles, independent one of the other, all equally fundamental, as regards our mind: in this case there will be not only one, but many resting-points. We do not believe it possible to determine _a priori_, whether there is unity or plurality for our intellect in this matter. That human science must be reduced to a single principle is a proposition that has been asserted, but never yet proved. Since the source of all truth, as has been shown in the preceding chapters, is not in man, it is evident that the principles, on which his knowledge is founded, must be communicated. Who shall assure us that they are not many in number, and of different orders? Nothing then in the present question can be resolved _a priori_, and we must descend to ideological and psychological observations. 146. Our mind acquires truth, or at least the appearance of truth; that is to say, that in one way or another, it performs those acts which we call perception and sensation. Whether the reality does or does not correspond to the acts of our soul, is at present of no consequence, is not what we now seek: we place the question on a ground accessible to the most skeptical; for even they do not deny perception and sensation; although they destroy reality, they admit appearance. 147. The means by which we perceive truth are of different orders; and this is why truths perceived correspond equally to different orders, parallel, so to speak, with the respective means of perception.