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45 "...the biosphere does not contain a predictable class of objects or events but is a particular event, certainly compatible indeed with first principles, but not *deducable* from those principles, and therefore unpredictable." (1) However, many scientists would argue that it would be possible to predict the occurence of life, if only we knew enough about biology. Those who believe that life is quite common in the universe often say that it is unlikely to be much like life on Earth: "Some scientists have been especially impressed by the number of individually unlikely events which are together responsible for the development of man and human intelligence. They have emphasised that even if the Earth were starting out again from scratch, and only random factors allowed to operate, the development of anything like a human being would be Wet. 2a ray highly unlikely." (2) Such opinions seem very plausible, but they fail to take into account the various constraints imposed by the laws of nature. For example: "...silicon compounds might replace carbon molecules as structural biochemicals". (3) The notion that there could be forms of life based on silicon rather than carbon crops up again and again in the literature. It should not be taken seriously, as such an idea betrays an ignorance of elementary chemistry. Some writers assume that because silicon is the nearest neighbour to carbon in the periodic table and also has a valency of four, then it can form bolds with hydrogen, oxygen, etc., to produce compounds analogous to compounds based on carbon and having similar properties. this is not true. The bond energies involved in the links between silicon and other elements, and carbon and other elements, have different values. the Si-Si bondis weaker than the C-C bond, but the Si-O bond is stronger that the C-O bond. To see what this means in practice we can compare the gas that bubbles out of fizzy drinks, carbon dioxide, with a lump of quartz, silicon dioxide. I maintain that anyone who takes the trouble to consult the appropriate chemistry text-books will be readily convinced that a biochemistry based on silicon is impossible. So here we have a basic constraint on the nature of any living creature, imposed by the laws of chemistry. Observational evidence indicates that these laws are the same throughout the observable universe, so there would seem to be no way of avoiding this conclusion. Assuming that intelligent beings have evolved on other planets, what would they look like? It is tempting to speculate that they would look like 'nothing on Earth’, but we must not let our imaginations run wild. The laws of nature impose many constraints on the size and shape of living organisms, and even on the social behavior of intelligent creatures. If we wish to design a hypothetical intelligent being, where do we start? Well, it would need a brain, and a very elaborate one at that. Thus it will need to be big enough to support a large brain, and this delicate organ will need some sort of protection. How about a skill? The brain will also need sensory inputs to tell it what is happening in the outside world. it will need eyes, of only to avoid such disasters as falling off cliffs or walking under buses. Yes, but would those eyes necessarily see the same wavelengths as our eyes? Undoubtedly so, because planetary atmospheres are most transparent to those wavelengths which we detect as visible light. What about 'seeing' with radio waves? Well, with radio waves: "In order to have any useful resolution - that is, detection of fine visual detail - the effective collecting area must be enormous. To have the same resolving power at 5cm wavelengths that they eye has at SOO0A. wavelength, an extraterrestrial microwave ‘eyeball’ would have to be roughly half a mile in diameter." (4) We could go on to fill a full-length book with similar arguments, but the whole question is neatly summed up in the words of Professor M R House: "Typical gross form associated with ecological habit for a given animal size and mode of life, suggests that there is a 'paradigm' or theoretically appropriate form for given circumstances, and that selection pressures tend to work towards this by eliminating those organisms least