The theory of “substantial and accidental in religions” divides all teachings of every religion into substantial and accidental. In this theory, the substantial part of a religion is defined as its content, essence, and destination, whereas the accidental part of it is its mould, husk, dust, and rust.
Like a proverb in which the apparent meaning of the words is different from its content, and the content is substantial and the appearance is accidental, and like Masnawi which has substantial and accidental parts, Islam has substantial and accidental parts. Since what makes Islam Islam is its substantial part, not the accidental one, you don't need to believe in or stick to accidental, and you can change them in every community according to its circumstances.
The only reason used to prove this theory is a false analogy. Moreover, the theorist commits the fallacy of begging the question at some points especially where he wants to give some criteria to distinguish between substantial and accidental. Some other problems of the theory are: inconsistent definitions of substantial and accidental, some misunderstandings of logical laws and drawing wrong conclusions from them, a misapplication of family resemblance theory, ascribing Ptolemaic spheres theory to Qur'an, and confusion between merely historical propositions and doctrinal ones.