Steven Pinker -- How the Mind Works - Hampshire High Italian ...

100 I HOW THE MIND WORKSincoming levels; if the total exceeds a threshold, the neuron will; becomemore active, sending a signal in turn to any neuron connected to it.Though neurons are always firing and incoming signals merely cause it tofire at a detectably faster or slower rate, it is sometimes convenient todescribe them as being either off (resting rate) or on (elevated rate).McCulloch and Pitts showed how these toy neurons could be wiredup to make logic gates. Logic gates implement the basic logical relations"and," "or," and "not" that underlie simple inferences. "A and B" (conceptually)is true if A is true and if B is true. An AND-gate (mechanically)produces an output if both of its inputs are on. To make an ANDigate outof toy neurons, set the threshold of the output unit to be greater thaneach of the incoming weights but less than their sum, as in the mini-networkon the left below. "A or B" (conceptually) is true if A is true or if B istrue. An OR-gate (mechanically) produces an output if either of its inputsis on. To make one, set the threshold to be less than each incomingweight, as in the middle mini-network below. Finally, "not A" (conceptually)is true if A is false, and vice versa. A NOT-gate (mechanically) producesan output when it receives no input, and vice versa. To make one,set the threshold at zero, so the neuron will fire when it gets no input,and make the incoming weight negative, so that an incoming signal willturn the neuron off, as in the mini-network on the right.6AND OR NOTSuppose that each toy neuron represents a simple proposition. Themini-networks can be wired together, with the output of one feeding theinput to another, to evaluate the truth of a complex proposition. Forexample, a neural network could evaluate the proposition {[(X chews itscud) and (X has cloven hooves)] or [(X has fins) and (X has scales)]}, asummary of what it takes for an animal to be kosher. In fact, if a networkof toy neurons is connected to some kind of extendable memory (such asa roll of paper moving under a rubber stamp and an eraser), it would be aTuring machine, a full-powered computer.©-.1l