As obtrusive examples of judgments of the common sense, Reid takes the logically undemonstrable convictions (1) of the existence of things external to and independent of me and my perceptions; (2) of my individual personal existence, and continuous personal identity; and (3) of the uniformity, and therefore interpretability, of nature, implying that I may form conclusions about what I have not experienced by what I have experienced. I cannot demonstrate the truth of these judgments: I can only justify them in reason by showing that human nature responds to them, and forbids me to reject them in my actions, on pain of being judged a lunatic, whatever I may say in my speculative theories. ‘Such original and natural judgments,’ he says, ‘are part of the furniture which _nature_ hath given to the human understanding. They are the inspiration of the Almighty. They serve to direct us in the common affairs of life, when our reasoning faculty would leave us in the dark; they are part of our constitution; all the discoveries of our reason are grounded upon them; they make up what is called _the common sense_ of mankind. A remarkable deviation from them, arising from a disorder in the constitution, is what we call _lunacy_, as when a man believes that he is made of glass. When a man suffers himself to be reasoned out of the principles of common sense by metaphysical arguments, we call it _metaphysical lunacy_, which differs from the other species of the distemper in this, that it is not continued, but intermittent; it is apt to seize the patient in solitary and speculative moments; but when he enters into sanity, Common Sense recovers her authority.’ It is the first of these three examples of the common sense that is the subject of the _Inquiry_—intended as preliminary to others which may follow. ‘A clear enumeration and explication of the principles of common sense’ was henceforward Reid’s ultimate aim; but thus far he took those involved in our experience when we are tasting, or smelling, or hearing, or touching, or seeing. As axioms and definitions are the preliminaries of abstract mathematical reasonings, so our sensuous perceptions through the five senses are the preliminaries to reasonings about concrete realities in the world of change. The leading principle of the _Inquiry_ is, that something extended, solid, and movable is directly manifested in our sensuous experiences, equally with the direct manifestations of our own sensations and other feelings when we are conscious of them; and also that we are obliged by common sense to regard extended, solid, and movable things as _other_ than mere transitory sensation or idea. Extended things, large or small, refuse to be melted down into sensations or states of one’s private consciousness. Take this specimen of philosophical argument for accepting perceptions which cannot be logically demonstrated, but which the common sense of human nature refuses to reject:—