When Ayn Rand distinguishes 'existence' from 'consciousness', she mainly means by "existence" what other philosophers call "the external world" -- thus, the distinction is between states of one's own mind and external phenomena.(1) According to Objectivism, existence has primacy over consciousness in two senses. First, epistemologically: human knowledge begins with (sensory) awareness of the external world. It does not begin with awareness of one's own ideas. The reason is that ideas or states of consciousness are necessarily ideas about something, and that something is what one is aware of. One could not become aware of one's own consciousness, unless one first had some states of consciousness to be aware of; and one could not have states of consciousness, unless one first had something else that one was conscious of.(2)
Second, existence has a metaphysical primacy over consciousness: that is, the external world exists prior to, and is not dependent on, our minds. This also implies that the external world has its specific characteristics (identity) prior to, and independent of, the states of our minds. And this is for the same reason: in order to have states of consciousness, there must first be things for us to become conscious of (whereas the converse is not the case), because consciousness is consciousness of things. Consciousness, again, is a faculty of becoming aware of things, not of creating or altering them.
Historically, most philosophers (especially in the modern era) have failed to recognize one or the other of these points and have upheld the contrary principle, the primacy of consciousness, in some form. Descartes is the clearest example of an epistemological primacy-of-consciousness thinker. Early in his Meditations, he lays down that he can be certain of the contents of his own mind -- his ideas, feelings, and sensations -- but there might not be any external objects. It is only the existence of his mind and thoughts that is self-evident, according to Descartes. Although he does eventually conclude as to the existence of an external world, he regards this fact as derived from facts about his own ideas. Nor is this notion that ideas are the only direct objects of awareness unique to Descartes; it is shared also by Locke, Hume, and Berkeley, among others.
G.W.F. Hegel is among the clearest exponents of the metaphysical primacy of consciousness. He holds that the entire universe is a consciousness, and that the events of history (including natural history) are all the results of the cosmic spirit's attempts at self-consciousness. So consciousness controls reality, and its activity is becoming conscious of, what? Itself.
William James also asserts the metaphysical primacy of consciousness, in a different way. Pragmatism's main idea is that truth is what is useful, or what 'works'. Now this implies that in a certain sense, reality waits on human needs and desires. If a proposition is not useful to us, then it won't be true. If we find it 'useful' to believe in God, then the belief will be true -- that can only mean that God will actually come into existence, consequent on our needing to believe in him. Now whether or not it is in fact useful to believe in God is not the point, of course. Even if all true beliefs are useful and all false beliefs are useless, the question remains whether beliefs are true because they are useful, or whether they are only useful because they are true. It is the former answer that represents the primacy of consciousness.
But the philosopher who most thoroughly maintains the primacy of consciousness in both the epistemological and metaphysical aspects is Immanuel Kant. On the one hand, he claims, not merely that our own ideas are the first things we are aware of, but that 'appearances' or ideas in the mind are the only things that anybody can ever know:
I say that things as objects of our senses existing outside us are given, but we know nothing of what they may be in themselves, knowing only their appearances, i.e., the representations which they cause in us by affecting our senses.(3)
This quotation makes it clear that 'appearances' are ideas in the mind.
Second, in the process of trying to validate synthetic a priori knowledge, Kant writes:
If intuition must conform to the constitution of the objects, I do not see how we could know anything of the latter a priori; but if the object (as object of the senses) must conform to the constitution of our faculty of intuition, I have no difficulty in conceiving such a possibility.(4)
The context makes it clear that Kant is asserting that we can have a priori knowledge, and thence concluding that objects must conform to the constitution of our faculty of 'intuition'. "Intuition", for Kant, means the direct knowledge of particular things; so what he is concluding is that the nature of external objects is constrained by the nature of our perceptual faculties.
Ancient philosophy was less wedded to the primacy of consciousness. The only place Plato directly addresses the issue is where he considers Protagoras' doctrine that "man is the measure of all things," at which point he portrays Socrates as refuting it.(5) Nevertheless, Rand would argue that Plato is also ultimately a primacy-of-consciousness thinker, because of his theory of the Forms. In the first place, Plato argues that the Forms must exist because of our ability to use concepts -- he argues that there must be a Form corresponding to each abstract noun that we can understand (e.g., "the good", "beauty", "unicorn") -- regardless of whether we perceive any referents of it. He uses facts about our mental abilities (even merely imaginative abilities) to derive significant metaphysical conclusions.(6) Second, he claims that the Forms are more real than the humdrum concrete objects we see around us, and that the Forms give those objects their identity (for instance, the Form of Man makes me human by entering into me). Rand would regard this as betokening the primacy of consciousness, because Plato's Forms are really hypostatizations of concepts -- so that what Plato is really doing is regarding concepts as more real than, and determining the natures of, other existences.
However, it should be noted that Plato did not assert the primacy of consciousness as such. If someone had asked him directly, "Does consciousness have primacy over existence?" he would have answered, "No. The Forms exist and are what they are prior to and independent of all observers." Plato is trying to uphold the primacy of existence. If anything, his problem is rather assigning too little role to consciousness than too much.
Finally, Aristotle is probably the thinker who most exemplifies the primacy-of-existence viewpoint prior to modern times. He, too, firmly criticized relativism. More importantly, he first laid down the law of identity, and he inaugurated the method of empirical science, involving the observation of nature (as against Plato's method of simply contemplating one's own ideas). Here is what he says against Protagoras and similar thinkers:
And, in general, if only the sensible exists, there would be nothing if animate things were not; for there would be no faculty of sense. Now the view that neither the sensible qualities nor the sensations would exist is doubtless true (for they are affections of the perceiver), but that the substrata which cause the sensation should not exist even apart from sensation is impossible. For sensation is surely not the sensation of itself, but there is something beyond the sensation, which must be prior to the sensation...(7)
which is almost a paraphrase of Rand, with "sensation" substituted for "consciousness".
Now Ayn Rand, of course, is the paradigmatic primacy-of-existence philosopher, and this basic stance runs throughout her thought. In the first place, she lays down "Existence exists" as the first fundamental axiom that we (implicitly) grasp, and it is only after we grasp this that, by taking cognizance of our own grasp of it, we can arrive at the second axiom, that our consciousness exists.
Second, because of this epistemic primacy of existence, she regards perception as the base of knowledge and as self-evident (for perception is the direct awareness of external existents). Whereas Kant argued that since a priori knowledge exists, it must be the case that reality conforms to our minds, Rand argues conversely: since reality does not have to conform to our minds (but rather the reverse is the case), there can not be a priori knowledge -- we can only acquire knowledge through interacting with the world.
Third, Rand regards reason as absolute (that is, as always to be used and never defied), because she recognizes the absoluteness of reality. To defy reason means, essentially, to set one's desires, feelings, or even arbitrary assumptions, against reality. The primacy of existence entails that such a course can only be self-destructive -- reality will not yield.
Fourth, Rand applies the idea of the primacy of existence to ethics. She argues that values are objective, rationally demonstrable, and derived from the nature of man and the requirements of human life. This is opposed, for example, to the doctrine that the good is whatever people want (per von Mises and various subjectivists), what pleases us (utilitarianism), what we have a sentiment of approval towards (Hume), or otherwise what we have certain feelings towards. All of those would be applications of the primacy of consciousness to ethics, insofar as they say that what is good for us depends on what consciousness considers good.
Objectivism takes the same approach to political philosophy, regarding government as a tool designed to serve specific, objective requirements of human life. Thus, what is just or unjust, and what rights people have, are facts rooted in human nature, not dependent on leaders' caprice (contra most pre-Enlightenment doctrines as well as contemporary legal positivism).
Finally, Objectivism treats aesthetic value as objective in the same way. Good art, again, is not what causes a certain 'aesthetic emotion' in us. Rather, it serves a rationally demonstrable requirement of the human psyche, the need for a concrete embodiment of one's views of the nature of life and man's place in the universe.
In all of these matters, Rand is seeking to put reality first, to uphold the existence of the relevant facts independent of our feelings and wishes. She is arguing that we must accept reality as we find it.
1. This is not to imply, obviously, that internal phenomena don't 'exist'. The point is just to distinguish consciousness from the rest of existence.
2. Ayn Rand, Introduction to Objectivist Epistemology (hereafter, ITOE) (New York: NAL Books, 1990) p. 246.
3. Prolegomena to any Future Metaphysics, remark II, following section 13.
4. Critique of Pure Reason, preface to second edition. Kemp Smith, tr., Bxvii.
5. Theaetetus, 177c-179b.
6. ITOE, p. 53.
7. Metaphysics, IV.5, 1010b30-37. W.D. Ross' translation.