 Similarly, it is important in understanding Lakatos’s theorizing to realize (for example) that in later life he wanted to develop a demarcation criterion between science and non-science that left Soviet Marxism (though not perhaps all forms of Marxism) on the non-scientific side of the divide. And this holds whether we regard this constraint as a non-rational external factor or as a constituent of his problem situation and hence internal to a rational reconstruction of his intellectual development. Biographical facts can be relevant to understanding a thinker’s ideas since they can help to illuminate the problem situation to which they were addressed.