 A suppositional theory was advanced by J. L. Mackie (1973, chapter 4). See also David Barnett (2006). Peter Gärdenfors’s work (1986, 1988) could also come under this heading. But the most fruitful development of the idea (in my view) takes seriously the last part of the above quote from Ramsey, and emphasises the fact that conditionals can be accepted with different degrees of closeness to certainty. Ernest Adams (1965, 1966, 1975) has developed such a theory. 