Keeping in mind these general truths, we shall be in a condition to understand certain causes of effect in composition now to be considered. Every perception received, and every conception realized, entailing some amount of waste--or, as Liebig would say, some change of matter in the brain; and the efficiency of the faculties subject to this waste being thereby temporarily, though often but momentarily, diminished; the resulting partial inability must affect the acts of perception and conception that immediately succeed.