Concerning justificatory standpoints, human agents have two: the first-person standpoint, which I take to be phenomenology in the Husserlian sense and the third-person standpoint of the sciences. In Thomas Nagel's What is it like to be a bat?, Nagel observes the difficulty with reconciling the first person phenomenology with the objective viewpoint. He supports the belief that what is needed is a phenomenology based on the third-person viewpoint.
At present we are completely unequipped to think about the subjective character of experience without relying on the imagination-without taking up the point of view of the experiential subject. This should be regarded as a challenge to form new concepts and devise a new method-an objective phenomenology not dependent on empathy or the imagination.1
In the same vain, others have moved to eliminate the first-person standpoint altogether. Some are eliminativists like Dennett and Churchland. Each sees the first-person as problematic and inconsistent in providing insights into the nature of why we must either abandon completely or simply believe in the first-person pragmatically. They feel that neuroscience can account for subjectivity. For instance, Churchland writes:
Our mutual understanding and even our introspection may then be reconstituted within the conceptual framework of completed neuroscience, a theory we may expect to be more powerful by far than the common-sense psychology it displaces, and more substantially integrated within physical science generally.2
What each of these moves entails is a view of the first-person as incapable of moving beyond the realm of the subjective and offering an exact theoretical picture of what exactly is consciousness in the fullest objective sense. Since the first-person standpoint is incapable of providing an objective account, there must be a problem in that it cannot reach beyond itself. This makes it incapable of other possibilities, namely, intersubjectively demonstrating objective knowledge to others or that the other exists. Solipsism is a consequence of the skepticism concerning the first-person standpoint from the naturalistic third-person standpoint.
Thus, there are two issues at work here; two issues I see as inseparable. First, I see Husserl's suspension of the naturalistic attitude as an answer to the problems of wanting to eliminate the subjective, first-person standpoint altogether. I see his criticism and the phenomenological reduction as a corrective measure against this eliminative impulse. Secondly, the charge of solipsism is interrelated since if one keeps to the naturalistic attitude, then one will eventually opt for the naturalist position regarding how to explain agency.
1Nagel, Thomas, “What is it like to be a bat?” The Philosophical Review 83, No. 4. (Oct., 1974), pp. 449.
2 Churchland, Paul, “Eliminative Materialism and Propositional Attitudes.” Journal of Philosophy 78, No. 2 (Feb., 1981): pp. 67