In the decades before the Origin, a split arises between two very different concepts of analogy, and so two views of argument by analogy. Some people, taking 'analogy' as a synonym for 'similarity',came to a new understanding of 'argument by analogy': suppose A and B are known to share a number of properties, then the probability is increased that B also possesses some other property which A is known to possess. This account remains widely assumed even today. Other people, largely within Anglican theology and concerned with the analogy between God and the world, insisted that the only correct use of the word 'analogy' was in its original Greek sense, including the Aristotelian commitment to proportionality, and so to relational comparisons, as when God is related to his creatures as a human father is to his children. This commitment grounded a view radically different from the new similarity view. In analysing what an argument by analogy is, Richard Whately, and following him J. S. Mill, specified explicitly the conditions for such an argument to be valid. It is this account that is relevant to an understanding of Darwin’s use of analogy.