With spacecraft on the way to Comet Halley likely to yield a flood of new knowledge, the present seems an appropriate time for those who advocate hypotheses about comets to stand up and be counted. This is especially so of those who, like the writer, are unable to accept aspects of the so-called received view.
The main questions concern the origin and ages of comets, their physical form and composition, and their orbits. It must be said that the author does not know the scientific answers to these questions, for the simple reason that science does not concern itself with absolute truth, but only with ascertaining which hypotheses have the greatest likelihood in the light of available evidence. Science regards a statement as meaningful only if it is capable of being tested operationally. Since a test would be nugatory unless it could fail, it follows that all scientific knowledge is in principle capable of refutation, and is therefore held only provisionally.
Scientific method consists essentially in abstracting, by observation or experiment, a description of an aspect of natural phenomena. Consequences of this description are then made explicit by theoretical development, and the result tested by confrontation with new observations. Two guiding principles are crucial. First, an hypothesis can be strengthened only by agreement with observations independent of those which originally suggested the hypothesis. In other words, only successful prediction counts; mere circular 'confirmation' is valueless. Secondly, an hypothesis is to be preferred in proportion as it accounts for the most facts with the least assumptions. In particular, a theory is worthless if a new assumption is needed to 'explain' each fact. This principle of parsimony of hypotheses is often known as the razor of William Occam (c. 128&1349) and rendered alternatively as 'Entia non sunt multiplicanda praeter necessitatem' or 'Pluralitas non est ponenda sine necessitate'. It is also referred to by quoting Isaac Newton (1642-1727) 'hypotheses non fingo'. Neither verbal assertions nor established authority has scientific standing; 'Nullius in verba'.
Of the main cometary questions, that of composition is, in one sense, the easiest and, in another sense, the least soluble. The observable part of a comet will consist mainly of cosmically abundant volatiles on almost any hypothesis whatsoever. Composition is therefore the least diagnostic characteristic of comets, and it is strange that it is sometimes quoted as if it were a clinching argument in favour of the 'icy snowball' hypothesis.