There is no doubt that ordinary men make their sensations so far objective as to transfer what passes within themselves to the exterior, without distinguishing between the subjective and the objective. Thus, when they consider colors to be in things themselves, they do not take the green, for example, to be the sensation of the green, but a thing certain, a quality, or whatever else it may be called, inherent in the object. But in reality this certainly is not so. The cause of the sensation is in the external object; such a disposition of facts also as to produce through the medium of light the impression called _green_. Common sense here deceives; for philosophical analysis convicts it of error. But here common sense has not the requisite conditions. In the first place, it ought to stand the test of reason; so soon as we reflect upon the case, we discover an illusion as fair as innocent. Moreover, it is not irresistible, for our assent is withheld the instant we are convinced of the illusion. Neither is the assent universal, for not all philosophers have it. Nor is it indispensable to the satisfaction of some great necessity of life. It therefore has no one of the conditions just laid down. What we have said of sight is applicable to the other senses. What is the value of the testimony of common sense inasmuch as it leads us to make sensations objective? We will answer this question. A certainty that sensations correspond to external objects is necessary to the wants of life. Upon this all men without exception are agreed. Reflection cannot despoil us of our natural inclination; and although reason, when most it cavils, may shake the foundations of this belief, it never succeeds in convicting it of error. Even they who give the most weight to such cavils, cannot prove that bodies do not exist: they can only say that we do not know that they exist. The natural inclination then possesses upon this point all the characteristics necessary to elevate it to the rank of an infallible criterion: it is irresistible, universal, satisfies a great necessity of life, and stands the test of reason.