[Constraint] is a relation between two sets, and occurs when the variety that exists under one condition is less than the variety that exists under a… - W. Ross Ashby

The most basic facts in biology are that this earth is now two thousand million years old, and that the biologist studies mostly that which exists today.

Many workers in the biological sciences — physiologists, psychologists, sociologists — are interested in cybernetics and would like to apply its methods and techniques to their own specialty. Many have, however, been prevented from taking up the subject by an impression that its use must be preceded by a long study of electronics and advanced pure mathematics; for they have formed the impression that cybernetics and these subjects are inseparable.
The author is convinced, however, that this impression is false. The basic ideas of cybernetics can be treated without reference to electronics, and they are fundamentally simple; so although advanced techniques may be necessary for advanced applications, a great deal can be done, especially in the biological sciences, by the use of quite simple techniques, provided they are used with a clear and deep understanding of the principles involved. It is the author’s belief that if the subject is founded in the common-place and well understood, and is then built up carefully, step by step, there is no reason why the worker with only elementary mathematical knowledge should not achieve a complete understanding of its basic principles. With such an understanding he will then be able to see exactly what further techniques he will have to learn if he is to proceed further; and, what is particularly useful, he will be able to see what techniques he can safely ignore as being irrelevant to his purpose.

The concept of "variety" [is] inseparable from that of "information."