By "subjective", I mean something that occurs only in a single mind and cannot be shared even in principle. A common example is pain. You cannot feel someone else's pain. You might observe some signs of pain in someone else, or empathize with their pain, but if you feel pain identical to what they are feeling, then it is your own pain and not theirs. Pain is subjective.
By "objective", I mean a thing that can be shared by multiple minds. Although my experience of a sunset may be subjective, you can experience the same sunset and that which is shared by both of us is something objective.
Clearly, objectivity is necessary for communication. I can't communicate my pain to you, but I can communicate to you the concept of pain and the proposition that I am feeling pain. By contrast, I can refer to a sunset that we both experienced, and then you and I are (assuming communication succeeds) both thinking of the same objective sunset. This need for objectivity, for something shared between minds, is why there have to be objective things corresponding to abstract objects. It is why abstract objects cannot be purely mental experiences. If the number three is just a thought in my head, then I can't share it with you any more than I can share my pain.
On the other hand, objectivity is often thought of, not merely as something that can be shared between minds, but also as a sign of, or a requirement of reality. I might have an experience of a sunset while I am dreaming. There is no objective sunset in this this case; it is not real. If it is real, then I can share that experience.
I have always assumed that because objectivity is bound up with reality, that the ability to distinguish the objective from the subjective is essential for practical interaction with the world, just as the ability to distinguish the real from the imaginary is necessary for practical interaction with the world. But now, I'm wondering if that assumption is correct. I can distinguish the real from the imaginary without appealing to the concepts of objective and subjective. I can conceive of a purely subjective mind, even a solipsistic mind, that has no thought of anything existing outside the mind. All experiences are just that--experiences. However, some experiences are real, meaning that they have ongoing consequences, while other experiences are imaginary, meaning that they vanish away, leaving no consequences behind.
For example, this mind might dream that there is a lion about to devour him, and wake in fear, and then realize that the dream was an experience with no consequences. But then a real lion appears, and the mind realizes that this experience is different from the dream in important ways. There are consequences that will follow from being caught by this lion--those consequences being negative experiences--so the solipsistic mind will avoid the real lion.
Is this right? Is objectivity only necessary for communication? Can an isolated hunter/gatherer safely be a solipsist? This would be a very startling result to me, but I'm starting to think that it may be true. It makes sense in a way, because the definitions of subjective/objective (the ones that I use) seem to only be relevant in a world where other minds exist and you need to interact with them.