What is experienced is the cause of the sensory perception involved in the experience. The external world most certainly exists, and without out it, there could be no negation. Negation is necessary for self awareness…as Sartre put it (in a non-cartesian way) the fundamental relationship to the world is through making a distinction between the “I” and the “that”. This is a negation of being, first and foremost, as the axiom that an “I” is not what it is aware of. It is called negative because it is not a Being itself, but the complete lack of being. The Cartesian second substance of mind is not a distinct ontological category itself, because it is expressed in the world as an epiphenomenal property of being. It is certainly a real process, consciousness that is, but it is not causally affectionate to the world of things. A “thought” does not affect the empirical world of forces and objects. It is instead an effect of a combination of material effects. As one of two modes of being, consciousness or mind is active insofar as it entertains an idea, and can get at nothing but an idea (does not reach the world causally or mechanically). These ideas are contingent to objective being, the only other mode of being and absolutely necessary for reality.
Although the sense data that affects us so that we respond to an environment is external to us, many of our ideas can be produced through means which do not involve direct immediate sensory experience. That is, in such things as memory, imagination, notion, and perhaps “intuition”, we conglomerate a world that moves about in our heads, full of imagery and words, thinking to ourselves, etc. But this is only a replication of what would be a real time experience, granted it does not involve impossible events.
Now I want you to consider this. We use terms which do not have as their object, things in the world. Not all words are nouns, in other words. The question then is how do such words obtain meaning, and unfortunately many philosophers like to think that just because many words represent ideas and not things, those things expressed with such terms are not inevitably reducible to external objective conditions.
In the case of the use of a word like “beauty” in a conversation, the term would be evoking a meaning that is improvised by the speaker, but be pragmatically grounded in real events, tacit agreements, and dramatic acts. This does indeed mean that at some point, an initial objective situation provided the possibility for such a term/concept to be formed. When someone had in mind the term “beauty”, they meant something very specific, but this doesn’t mean that the term must be understood as a term which can have an objective meaning. That is impossible. We know through deduction that material circumstances must precede the development of all instances in language, although every term takes on an improvisational meaning when being used.
The term “beauty”, therefore, cannot mean anything other than the sum total of events which were occurring when the term was/is being invented/used.
If it could mean something else, it is only because it has been used in new circumstances, but this does not detract from its original inception. Despite what the speaker meant when he made up the term “beauty”, there were very certain objective material conditions present when he did.
So even though ideas can be composed of terms which are not existentially quantifiable, such as adjectives, those terms have origin, use, and function in responding and subordinating the environment, the “world”.
Phenomenology claims that both Empiricism and Rationalism are erroneous. Reality is not “a priori” to experience, as representationalists such as Schopenhauer and Kant claim, and neither is it “posterior” to experience, as Locke and Hume claim. There is an objective reality which exists prior to experience but its meaningfulness cannot be interpreted without corresponding to intentional acts.
1+1=2 is an indifferent objective fact before it is experiences as a symbolic representation of objects in the world. But when it is actualized in consciousness and mediated in thought, it is an intentional structure used in a context of activity in the world. Math means something because as such symbol sets it represents real facts about the world which are not contingent to experience, but which don’t have any necessary meaning. They are indifferent ontological facts. As such, they are not yet a “reality” until they are experienced.