The heart of animals is the foundation of their life, the sovereign of everything within them, the sun of their microcosm, that upon which all growth… - William Harvey

Very many maintain that all we know is still infinitely less than all that still remains unknown; nor do philosophers pin their faith to others' precepts in such wise that they lose their liberty, and cease to give credence to the conclusions of their proper senses. Neither do they swear such fealty to their mistress Antiquity that they openly, and in sight of all, deny and desert their friend Truth. But even as they see that the credulous and vain are disposed at the first blush to accept and believe everything that is proposed to them, so do they observe that the dull and unintellectual are indisposed to see what lies before their eyes, and even deny the light of the noon-day sun.

As art is a habit with reference to things to be done, so is science a habit in respect to things to be known.

Good God! how should the mitral valves prevent the regurgitation of air and not of blood?