Every language is both a _scheme for expressing meanings_ and _physical equipment_ that can be handled. For example, let us take _spoken English_. The scheme of spoken English consists of more than 150,000 words expressing meanings, and some rules for putting words together meaningfully. The physical equipment of spoken English consists of (1) sounds in the air, and (2) the ears of millions of people, and their mouths and voices, by which they can hear and speak the sounds of English. For another example, let us take numbers expressed in the _Arabic numerals_ and the rules of arithmetic. The scheme of this language contains only ten digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or their equivalents, and some rules for combining them. Sufficient physical equipment for this language might very well be a ten-column desk calculating machine with its counter wheels, gears, keys, etc. If we tried to exchange the physical equipment of these two languages, we would be blocked: the desk calculating machine cannot possibly express the meaningful combinations of 150,000 words, and sounds in the air are not permanent enough to express the steps of division of one large number by another. SCHEMES FOR EXPRESSING MEANINGS If we examine languages that have existed, we can observe a number of schemes for expressing meanings. In the table on pp. 12-13 is a rough list of a dozen of them. From among these we can choose the schemes that are likely to be useful in mechanical brains. Schemes 11 and 12 are the schemes that have been predominantly used in machinery for computing. Scheme 12 consisting of combinations of just two marks, ✓, ✕, provides one of the best codes for mechanical handling of information. This scheme, called _binary coding_ (see Supplement 2), is also useful for measuring the quantity of information. QUANTITY OF INFORMATION How should we measure the quantity of information? The smallest unit of information is a “yes” or a “no,” a check mark (✓) or a cross (✕), an impulse in a nerve or no impulse, a 1 or a 0, black or white, good or bad, etc. This twofold difference is called a _binary digit_ of information (see Supplement 2). It is the convenient _unit of information_. SCHEMES FOR EXPRESSING MEANINGS EXAMPLE: /————————————————^—————————————————\ PRINCIPLE SIGN USED IN SIGNIFICANCE NAME OF NO. OF SCHEME SCHEME (1) (2) (3) (4) (5) (6) _Sounds_ 1. Sound of new Bobwhite[2] Spoken kind of quail, Imitative; word is like English so called bowwow sound of from its note theory referent