The visible phenomena of the universe are bound by the universal law of cause and effect. The effect is visible or perceptible, while the cause is invisible or imperceptible. The falling of an apple from a tree is the effect of a certain invisible force called gravitation. Although the force cannot be perceived by the senses, its expression is visible. All perceptible phenomena are but the various expressions of different forces which act as invisible agents upon the subtle and imperceptible forms of matter.