http://plato.stanford.edu/entries/leibniz-mind/
Leibniz's point is that whatever is the subject of perception and consciousness must be truly one, a single “I” properly regarded as one conscious being. An aggregate of matter is not truly one and so cannot be regarded as a single I, capable of being the subject of a unified mental life. This interpretation fits nicely with Lebniz's oft-repeated definition of perception as “the representation in the simple of the compound, or of that which is outside” (Principles of Nature and Grace, sec.2 (1714)). More explicitly, in a letter to Antoine Arnauld of 9 October 1687, Leibniz wrote that “in natural perception and sensation, it is enough for what is divisible and material and dispersed into many entities to be expressed or represented in a single indivisible entity or in a substance which is endowed with genuine unity.” If perception (and hence, consciousness) essentially involves a representation of a variety of content in a simple, indivisible “I,” then we may construct Leibniz's argument against materialism as follows: Materialism holds that matter can explain (is identical with, can give rise to) perception. A perception is a state whereby a variety of content is represented in a true unity. Thus, whatever is not a true unity cannot give rise to perception. Whatever is divisible is not a true unity. Matter is infinitely divisible. Hence, matter cannot form a true unity. Hence, matter cannot explain (be identical with, give rise to) perception. If matter cannot explain (be identical to, give rise to) perception, then materialism is false. Hence, materialism is false.