Delving a bit deeper into Objectivity in this 121st post, a definition which was requested and provided as the 41st post in this thread, a complimentary passage can be found in Who Is The Final Authority In Ethics.
It is obvious that the root of such questions ["Is it intellectual plagiarism to accept and even to use philosophical principles and values discovered by someone else?"] is a certain kind of conceptual vacuum: the absence of the concept of objectivity in the questioner's mind.
Objectivity is both a metaphysical and an epistemological concept. It pertains to the relationship of consciousness to existence. Metaphysically, it is the recognition of the fact that reality exists independent of any perceiver's consciousness. Epistemologically, it is the recognition of the fact that a perceiver's (man's) consciousness must acquire knowledge of reality by certain means (reason) in accordance with certain rules (logic). This means that although reality is immutable and, in any given context, only one answer is true, the truth is not automatically available to a human consciousness and can be obtained only by a certain mental process which is required of every man who seeks knowledge—that there is no substitute for this process, no escape from the responsibility for it, no shortcuts, no special revelations to privileged observers—and that there can be no such thing as a final "authority" in matters pertaining to human knowledge. Metaphysically, the only authority is reality; epistemologically—one's own mind. The first is the ultimate arbiter of the second.
The concept of objectivity contains the reason why the question "Who decides what is right or wrong?" is wrong. Nobody "decides." Nature does not decide—it merely is; man does not decide, in issues of knowledge, he merely observes that which is. When it comes to applying his knowledge, man decides what he chooses to do, according to what he has learned, remembering that the basic principle of rational action in all aspects of human existence, is: "Nature, to be commanded, must be obeyed." This means that man does not create reality and can achieve his values only by making his decisions consonant with the facts of reality.
This provides some rationale why volitional adherence to reality is prudent. It does not cover the fact that spectacular views of the Atlantic Ocean or of the Rocky Mountains exist. It does point out that in any given context, only one answer is true, thus trying to decide in a vacuum, i.e., trying to make a contextless decision, would be a departure from objectivity.
If you do lift the corner of the aforementioned rug, could you check to see if this was inadvertently swept under it as well?
"Do you cry that you find no answers? By what means did you hope to find them? You reject your tool of perception—your mind—then complain that the universe is a mystery. You discard your key, then wail that all doors are locked against you. You start out in pursuit of the irrational, then damn existence for making no sense.
— This is John Galt Speaking