"Self-production: the characteristic of living systems to continuously renew themselves and to regulate this process in such a way that the integrity of their structure is maintained. It is a natural process which supports the quest for structure, process renewal and integrity."

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To the pure geometer the radius of curvature is an incidental characteristic - like the grin of the Cheshire cat. To the physicist it is an indispensable characteristic. It would be going too far to say that to the physicist the cat is merely incidental to the grin. Physics is concerned with interrelatedness such as the interrelatedness of cats and grins. In this case the "cat without a grin" and the "grin without a cat" are equally set aside as purely mathematical phantasies.

— Arthur Eddington