Human intelligence is one of the most important yet controversial topics in the whole field of the human sciences. It is not even agreed whether it can be measured or, if it can, whether it should be measured. The literature is enormous and much of it is highly partisan and, often, far from accurate.

To justify a further incursion into this field it may help to think of the sporting analogy provided by professional football, of whatever code. There are many people who take a passing interest in the game; they may look up scores in the newspaper, but have little specialist interest and they would certainly not feel deprived if they were cut off from the game altogether. Then there are those we may call spectators, who follow the game closely. They may attend matches and watch games on television. Some will be violently partisan, cheering on their own team, failing to see the fouls on their opponents and hurling occasional abuse at the referee. Others will be connoisseurs who understand the niceties of the game and delight in the skills and artistry of the players. Only a few people will actually be engaged in the game as players. They are the ones who have a good technical knowledge of the rules but, more importantly, have the outstanding skills which enable them to perform well at the professional level. For them it is more than just a game, it is also the source of their livelihood.