 Some modern philosophy tries to resolve the problem of man by, in effect, reducing the transcendental to the empirical. For example, naturalism attempts to explain knowledge in terms of natural science (physics, biology), while Marxism appeals to historical social sciences. (The difference is that the first grounds knowledge in the past—e.g., an evolutionary history—whereas the second grounds it in a revolutionary future that will transcend the limitations of ideology.) Either approach simply ignores the terms of the problem: that man must be regarded as irreducibly both empirical and transcendental.