3. We must take the constitution of the human mind as we find it, and not as we may judge it ought to be;—thus I am led on to another remark, which is at first sight disadvantageous to Certitude. Introspection of our intellectual operations is not the best of means for preserving us from intellectual hesitations. To meddle with the springs of thought and action is really to weaken them; and, as to that argumentation which is the preliminary to Certitude, it may indeed be unavoidable, but, as in the case of other serviceable allies, it is not so easy to discard it, after it has done its work, as it was in the first instance to obtain its assistance. Questioning, when encouraged on any subject-matter, readily becomes a habit, and leads the mind to substitute exercises of inference for assent, whether simple or complex. Reasons for assenting suggest reasons for not assenting, and what were realities to our imagination, while our assent was simple, may become little more than notions, when we have attained to certitude. Objections and difficulties tell upon the mind; it may lose its elasticity, and be unable to throw them off. And thus, even as regards things which it may be absurd to doubt, we may, in consequence of some past suggestion of the possibility of error, or of some chance association to their disadvantage, be teazed from time to time and hampered by involuntary questionings, as if we were not certain, when we are. Nay, there are those, who are visited with these even permanently, as a sort of _muscæ volitantes_ of their mental vision, ever flitting to and fro, and dimming its clearness and completeness—visitants, for which they are not responsible, and which they know to be unreal, still so seriously interfering with their comfort and even with their energy, that they may be tempted to complain that even blind prejudice has more of quiet and of durability than certitude.