There may be truths shared by both, but points on which they differ and part ways, or they may be in total opposition. What is the best contending philosophy that has consistently contended with the points Objectivism raises?
Why does Objectivism depart from them or them from Objectivism?
That's very broad question. An equally broad answer is: all of them.
For the same reasons that any two philosophies differ: different views on metaphysics, epistemology, ethics, politics and esthetics.
answered Jan 08 '12 at 05:53
The literature that records Objectivism by both Ayn Rand and Leonard Peikoff highlight the two main contrasting philosophic theories (intrinsicism and subjectivism) along with some variations on their essential characteristics.
To understand how Objectivism's theory of concepts are derived from the perceptually given and use this given as a datum to refer the conceptual reckoning back to, and how the other philosophies try to take an alleged shortcut to knowledge, or posit that existence is somehow a product of consciousness - you might consider familiarizing yourself with the arguments presented by these two in their full presentation.
The major benefit of this suggestion would be to examine and validate for yourself what has already been discovered saving the time of the alternative: discovering and validating for yourself a philosophy from scratch.
answered Dec 28 '11 at 21:22