http://www.elpais.com/articulo/sociedad/Thinking/outside/the/laws/of/thought/elpepusoc/20110805elpepusoc_6/TesIn ancient Greece, Aristotle took some important first steps towards systematizing types of arguments where reasoning is perfectly reliable. To give an example: If all philosophers are human and all humans are mortal, then all philosophers are mortal. Who could accept the two premises but not the conclusion? Aristotle's discovery was not that this or that particular argument is valid, but that certain kinds of argument are valid: no matter what words are substituted for 'philosophers', 'human' and 'mortal' in the argument above, we get a valid argument.
Since Aristotle, logicians have identified many more such laws of thought. In addition, the branch of mathematics concerned with probability has extended these laws with rules for how to reason using uncertain premises. One such rule is the conjunction rule which states that a statement of the form 'A and B' cannot be more probable than one of its parts. So, for example, if we know how probable 'It will be sunny', is we can correctly conclude that 'It will be sunny and birds will sing' is not more probable.
Our knowledge of correct reasoning has come a long way since ancient Greece and its fruits can be found throughout the sciences?from mathematics to the empirical sciences. But have logic and statistics also given us the principles whereby we actually reason? Unfortunately not. Research in cognitive psychology during the past thirty years has convincingly demonstrated that humans habitually break even the most basic of the laws of logic and statistics.
Suppose you meet Linda who is 31 years old, outspoken and very bright. As a student, she was deeply concerned with issues of discrimination and social justice. Now, what would you say is more probable, a) that she is a bank teller, or b) that she is a bank-teller who is also a feminist?
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