Ranking Theory is a theory in formal epistemology that represents the agent's epistemic state by a ranking function. It could represent the qualitative and quantitative nature of belief. The theory was made by the German philosopher, Wolfgang Spohn, and it is discussed in his Lakatos award winner bookthe laws of belief. Here is where Spohn explains how the ranking theory was bornSpohn , Wolfgang, 2012, The Laws of Belief: Ranking Theory and Its Philosophical Applications, Oxford: Oxford University Press : 'Around 1980 Peter Gärdenfors’ papers on conditionals and belief change (1978, 1981) provided the best available account of conditional belief (besides Ellis 1979, which I was unaware of at that time), and this account appeared to offer more intelligible grounds for a theory of conditionals and causation than the similarity spheres used by David Lewis as a basis for those topics. However, I thought that Gärdenfors’ account represented conditional beliefs and the dynamics of belief only in an incomplete way, too incomplete for the purposes of a theory of causation. For months I struggled to find a way of completing the account. Somehow the solution came to my mind in April 1982, and what was later called ranking theory was born.'
Ranking theory is a theory in formal epistemology that represents the agent's epistemic state by a ranking function. It could represent the qualitative and quantitative nature of belief. The theory was made by the German philosopher, Wolfgang Spohn, and it is discussed in his book, The Laws of Belief. Spohn explains how the ranking theory was born:Spohn , Wolfgang, 2012, The Laws of Belief: Ranking Theory and Its Philosophical Applications, Oxford: Oxford University Press : 
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'Around 1980 Peter Gärdenfors’ papers on conditionals and belief change (1978, 1981) provided the best available account of conditional belief (besides Ellis 1979, which I was unaware of at that time), and this account appeared to offer more intelligible grounds for a theory of conditionals and causation than the similarity spheres used by David Lewis as a basis for those topics. However, I thought that Gärdenfors’ account represented conditional beliefs and the dynamics of belief only in an incomplete way, too incomplete for the purposes of a theory of causation. For months I struggled to find a way of completing the account. Somehow the solution came to my mind in April 1982, and what was later called ranking theory was born.'