• Can formal rules be used to draw valid conclusions?
• How does the mind arise from a physical brain?
• Where does knowledge come from?
• How does knowledge lead to action?
Aristotle (384–322 B.C.), whose bust appears on the front cover of this book, was the first to formulate a precise set of laws governing the rational part of the mind. He developed an informal system of syllogisms for proper reasoning, which in principle allowed one to generate conclusions mechanically, given initial premises. Much later, Ramon Lull (d. 1315) had the idea that useful reasoning could actually be carried out by a mechanical artifact. Thomas Hobbes (1588–1679) proposed that reasoning was like numerical computation, that “we add and subtract in our silent thoughts.” The automation of computation itself was already well under way. Around 1500, Leonardo da Vinci (1452–1519) designed but did not build a mechanical calculator; recent reconstructions have shown the design to be functional. The first known calculating machine was constructed around 1623 by the German scientist Wilhelm Schickard (1592–1635), although the Pascaline, built in 1642 by Blaise Pascal (1623–1662), is more famous. Pascal wrote that “the arithmetical machine produces effects which appear nearer to thought than all the actions of animals.” Gottfried Wilhelm Leibniz (1646–1716) built a mechanical device intended to carry out operations on concepts rather than numbers, but its scope was rather limited. Leibniz did surpass Pascal by building a calculator that could add, subtract, multiply, and take roots, whereas the Pascaline could only add and subtract. Some speculated that machines might not just do calculations but actually be able to think and act on their own. In his 1651 book Leviathan, Thomas Hobbes suggested the idea of an “artificial animal,” arguing “For what is the heart but a spring; and the nerves, but so many strings; and the joints, but so many wheels.”
It’sonethingtosaythatthemindoperates, atleastinpart, according tologicalrules,and to build physical systems that emulate some of those rules; it’s another to say that the mind itself is such a physical system. Ren´e Descartes (1596–1650) gave the first clear discussion of the distinction between mind and matter and of the problems that arise. One problem with a purely physical conception of the mind is that it seems to leave little room for free will: if the mind is governed entirely by physical laws, then it has no more free will than a rock “deciding” tofalltowardthe centerofthe earth. Descartes wasastrong advocate ofthepower of reasoning in understanding the world, a philosophy now called rationalism, and one that counts Aristotle and Leibnitz as members. But Descartes was also a proponent of dualism.
He held that there is a part of the human mind (or soul or spirit) that is outside of nature, exempt from physical laws. Animals, on the other hand, did not possess this dual quality; they could be treated as machines. An alternative to dualism is materialism, which holds that the brain’s operation according to the laws of physics constitutes the mind. Free will is simply the way that the perception of available choices appears to the choosing entity.
Given a physical mind that manipulates knowledge, the next problem is to establish the source of knowledge. The empiricism movement, starting with Francis Bacon’s (1561–EMPIRICISM 1626) Novum Organum,2 is characterized by a dictum of John Locke (1632–1704): “Nothing is in the understanding, which was not first in the senses.” David Hume’s (1711–1776) A Treatise of Human Nature (Hume, 1739) proposed what is now known as the principle of induction: that general rules are acquired by exposure to repeated associations between theirelements. Building on the work of Ludwig Wittgenstein (1889–1951) and Bertrand Russell (1872–1970), the famous Vienna Circle, led by Rudolf Carnap (1891–1970), developed the doctrine of logicalpositivism. Thisdoctrine holds thatallknowledge canbecharacterized by logical theories connected, ultimately, to observation sentences that correspond to sensory inputs; thuslogical positivismcombines rationalism andempiricism.3 Theconfirmationtheory of Carnap and Carl Hempel (1905–1997) attempted to analyze the acquisition of knowl-CONFIRMATION THEORY edge from experience. Carnap’s book The Logical Structure of the World (1928) defined an explicit computational procedure for extracting knowledge from elementary experiences. It was probably the first theory of mind as a computational process.
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