Material Method, Voluntary Action, Socialism

As Kant argues, mere observation alone does not suffice to distinguish some behavior as either 'voluntary' or 'involuntary'.  But, the more important application of the thesis is to internal observation, i. e. to introspection, which, as Spinoza, also, contends, cannot alone distinguish a motivating idea that is the product of conditioning, from one that is freely adopted.  Now, Marx and Engels implicitly agree with them when noting that an "illusory" motivation can be taken as a "real" one.  But, then they are acknowledging that their "material method", observation of concrete practice, lacks the capacity to distinguish voluntary from involuntary action.  Hence, they cannot recognize Socialism when they see it, and, furthermore, can derive no concept of Socialism from that method.