The importance of drawing a broad line between the active and the passive branches of our primary sensibilities is seen in various applications, but most especially in the problem of {5} External Perception. The great distinction that this problem requires us to draw between the external and the internal sides of our being (so described by an imperfect metaphor) has its deepest foundation in the distinction between the sense of expended muscular energy and the feelings that are neither energy in themselves, nor vary definitely according to our energies. The qualities of things admitted on all hands to be qualities of the external (or object) world--called the Primary Qualities, Resistance and Extension,--are modes of our muscular energies; the qualities that do not of themselves suggest externality, or objectivity,--the secondary qualities, as Heat, Colour, &c.--are our passive sensibilities, and do not contain muscular energy. When these secondary qualities enter into definite connections with our movements, they are then referred to the external, or object world. Light and colour, when varying definitely with our various movements, as postures and actions, are from that circumstance referred to the external, or _non-ego_; without such connections they would be called internal or subjective states. The contrasted terms 'Object' and 'Subject' are the least exceptionable for expressing the fundamental antithesis of consciousness and of existence. Matter and Mind, External and Internal, are the popular synonyms, but are less free from misleading suggestions. Extension is the Object fact by pre-eminence; Pleasure and Pain are the most marked phases of pure Subjectivity. Between the consciousness of extension and the consciousness of a pleasure there is the broadest line that can be drawn within the human experience; the broadest distinction in the whole universe of being. These then are the Object and Subject extremes; and, in the final analysis, the object extreme appears to be grounded on the feeling of expended muscular energy.