"The cause of nutrition and growth resides not in the organism as a whole but in the separate elementary parts-the cells."

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Medicine deals with the states of health and disease in the human body. It is a truism of philosophy that a complete knowledge of a thing can only be obtained by elucidating its causes and antecedents, provided, of course, such causes exist. In medicine it is, therefore, necessary that causes of both health and disease should be determined.

— Avicenna