I am trying to find the basic list of postulates that lay the foundation for QM, but i see a different list of postulates in different textbooks and different places. In MIT lectures, Prof Allan Adams gives 3 basic postulates: 1. State of a system given by Wave function. 2. Mod squared psi gives probability density. 3. superposition principle. In quantum mechanics by Shankar, i see 4 postulates: 1. Wave function and its meaning. 2. Definition of position and momentum operators and other operators. 3. Measurement of observable gives one of the eigenvalues of its corresponding operator. 4. Wave function satisfies Schrodinger equation. Online I see in may websites a list of 6 postulates which include statements on the orthogonality of eigenstates and method of finding expectation values, for instance http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/qm.html This is confusing as opposed to other subjects,say, special theory of relativity which always starts with 2 postulates from which everything else is built up. What is the most basic list of postulates in QM from which everything else will follow?