The great scientific achievement of the XIXth century consists in the victory over speculation, over knowledge without sense perception, in the delivery of the senses from the thraldom of such knowledge, and in the foundation of empirical investigation. To acknowledge the theoretical value of this achievement means to come to an understanding about the source of error. Contrary to a philosophy that tries to discover truth with the mind, and error with the senses, we seek for truth with the senses and regard the mind as the source of errors. The belief in certain messages of the nerves which are alone worthy of confidence and which can be understood only by degrees without any specific mark of distinction, is a superstition. Let us have confidence in all testimonials of the senses. There is nothing false to be separated from the genuine. The supernatural mind idea is the only deceiver whenever it undertakes to disregard the sense perceptions, and, instead of being the interpreter of the senses, tries to enlarge their statements and repeat what has not been dictated. The eye, in seeing forked lightning or radiant circles when the blood is excited or a pressure exerted on it, perceives no more errors than it does in perceiving any other manifestation of the external world. It is our faculty of thought which makes a mistake, by regarding without further inquiry such subjective events as objective bodies. One who sees ghosts does not commit any mistake, until he claims that his personal apparition is a general phenomenon, until he prematurely takes something for an experience which he has not experienced. Error is an offense against the law of truth which prescribes to our consciousness that it must remember the limits within which a perception is true, or general. Error makes out of something special a generality, out of a predicate a subject, and takes the part for the whole. Error makes _a priori_ conclusions, while truth, its opposite, arrives at understanding by _a posteriori_ reasoning. _A priori_ and _a posteriori_ understanding are related in the same way as philosophy and natural science, taking the latter in the widest meaning of the term, that of science in general. The contrast between believing and knowing is duplicated in that between philosophy and natural science. Speculative philosophy, like religion, lives on faith. The modern world has transformed faith into science. The reactionists in politics who demand that science retrace its steps desire its return to faith. The content of faith is acquired without exertion. Faith makes _a priori_ perceptions, while science arrives at its knowledge by hard _a posteriori_ study. To give up faith means to give up taking things easy. And to confine science to _a posteriori_ knowledge means to decorate it with the characteristic mark of modern times, work.