My concern is with the SSm. khyan description of the world, mind and spirit as preserved principally in the $5 .mkhya K5rik~s; the system of the 25 tattvas. The 'understanding' of my title refers not to the historical problem of where the S]mkhyas i~flaerited their ideas from, but rather to the problem of justification; what reasons could the Sam. khyas have had for thinking as they did. This is of course itself partly a historical problem, concerned with arguments which, according to the texts, they themselves gave. However, understanding their reasons involves thinking them through critically, making sense of them, so that their reasons come to be seen as the kind of reasons which might influence us. This may be achieved either by putting their reasons in terms of what seems reasonable to us, or by extending our ideas of what is reasonable to embrace their insights and ways of thinking.
For what reasons then should anyone, including the S~9. khyas themselves, believe in the system of the 25 tattvas, or what I shall call 'the tattva theory'? In this paper, which can be no more than an outline, I shall consider three types of reason, which I take to be the most important: