offered as solutions to today’s insoluble problems. The case for SETI, it must be stressed, rests on its appeal to plausibility which is based on knowledge derived from state-of-the-art technology. Confidence in SETI’s chance of success is predicated on developmentsin astronomy, space exploration, information technology, and the life sciences which offer a sound empirical foundation for SETI research.

There are, however, several logical pitfalls in the arguments which frequently appear in SETI literature; and philosophers of science have drawn attention to them. Ernan McMullan (1971) suggests that two types of probability may be sometimes confused in arguments concerning the likelihood of extraterrestrial life. The first type is inductive probability which is based upon a frequency count of outcomes: this might involve a count of heads or tails in a coin-tossing experiment or predictions based on a correlation between traffic accidents and teenage motorists. Conclusions drawn from inductive probability require neither an understanding nor an explanation of the processes involved. Thus a correlation may be made between teenage motorists and traffic accidents without recourse to any explanations why teenagers are involved in more or less accidents than other motorists. The second type of probability, theoretical probability, does require at least a partial understanding of the causes. Thus if one has a reasonable theoretical explanation of the psychological factors affecting teenage motorists, one could attempt a calculation of the percentage of likely accidents without observing numerous instances. Now appeals to inductive probability are of little value to SETI as we only know of one planet where intelligent life has evolved. But appeals to theoretical probability are not without methodological problems: although theoretical progress is underway, we are not yet in a position to advance a definitive theory as to how planets, life and then intelligence, came about. This lack of a firm theoretical base, argues McMullan (1971), is fundamentally damaging to the SETI enterprise and partly accounts for four fallacies which he detects in SETI literature.

The first fallacy, according to McMullan, is the belief that given enough time ‘the probability of any natural outcome dependent on universal natural processes increases to virtual certainty’ (ibid.: 292). For example, given enough time, any environment containing the constituents associated with life will inevitably produce life. The problem with this argument is that, lacking a theory of how life came about, it appeals to nothing more than a random juxtaposition of elements. But this is unlikely, as what we do know about life is that it emerges out of a gradual biochemical process involving, as McMullan says, ‘a host of unknown interdependent environmental factors’ (ibid.). The problem is not resolved by arbitrarily lengthening the timescale, but rather by knowing which of the essential factors will be present and what forces will be operating upon them.

The second fallacy McMullan identifies is the ‘uniformitarian fallacy’. This fallacy is found in the argument which states that if life can develop in one place it must inevitably develop in another place with similar conditions. This argument is sometimes linked with appeals to general scientific beliefs such as the

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belief that nature does not admit uniqueness. Again, McMullan identifies the fallacy with reference to our lack of a theory as to how life came about: for unless we have this theory for one environment we can have no idea how life will emerge elsewhere. The same appeal to the lack of a theoretical account of the emergence of life underpins McMullan’s depiction of the third SETI fallacy, the appeal to large numbers, the vast numbers of galaxies, stars and possible life-sustaining planets. Unless we have a theory of some sort we cannot attach any theoretical probability to any predicted outcome. It might be said that it is logically possible that life can emerge elsewhere, but expressions like ‘logically possible’ cannot be assigned a numerical rating if there is no theory to support it.

It might be countered that evolutionary theory provides a theoretical grounding for speculations about extraterrestrial life. But McMullan cites SETI’s appeal to evolutionary theory as an example of the fourth fallacy; namely that it is fallacious to use the theory of evolution as a predictive theory. SETI scientists often argue that once life originates the operation of natural selection will inevitably lead to more complex life-forms and ultimately to consciousness and intelligence. Yet biologists constantly point out that no predictions can be made on the basis of evolutionary theory; intelligence may bestow selective advantage, but its emergence cannot be guaranteed as other features may turn out to be more advantageous.

How damaging are these alleged fallacies in the reasoning of SETI expo-nents? They only appear to be damaging if the  arguments which contain them are employed to support an a priori claim that ETI exists. If, however, they are used in the context of discovery, as steps in a campaign for exploration, as reasons towards the plausibility of conducting an empirical investigation, they cannot be faulted. A rebuttal of the four ‘fallacies’ might read as follows:

1. A useful realistic strategy could rest on the belief that the chance of success is increased with an increase in the time available.
   
2. Uniformitarianism is only fallacious if one insists that uniform conditions must yield uniform results without exceptions; in many cases it is a good rule of thumb to seek similar results from similar conditions.
   
3. Likewise, an appeal to large numbers, while providing no guarantee for any particular prediction, is suggestive that a wider search may be productive.
   
4. Moreover, if claims regarding the inevitable emergence of intelligence are replaced with suggestions that intelligence is a likely development in some life-forms, for which it is selectively advantageous, then no harm is done to the non-predictive status of evolutionary theory.
   

It is important, therefore, to disentangle arguments which indicate that an empirical search might be fruitful from arguments which attempt to establish the existence of ETI on a priori grounds.

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