But this statism requires as its complement a moderate dynamism, and this is supplied by the central concepts of act and potency. Whatsoever changes is, just for that reason, limited. The oak-tree passes through a process of growth, of becoming: whatever is actually in it now was potentially in it from the beginning. Its vital functions go on unceasingly (accidental change); but the tree itself will die, and out of its decayed trunk other substances will come forth (substantial change). The theory of matter and form is simply an interpretation of the substantial changes which bodies undergo. The union of matter and form constitutes the essence of concrete being, and this essence is endowed with existence. Throughout all change and becoming there runs a rhythm of finality; the activities of the countless substances of the universe converge towards an end which is known to God; finality involves optimism.