The Penguin Dictionary of Philosophy defines solipsism, in the ontological sense, as the view that nothing exists except one's own self and the contents of its consciousness. The word is also used epistimologically to describe the claim that nothing can be known but oneself and the contents of one's consciousness.
An obvious response to the notion of solipsism might be that others clearly exist, whether we can know them or not; or again, that because one can discern the observable universe, objects within it are necessarily real and existent. It is easy to assume that because we, as humans, can observe others with similar qualities to our own, that others exist; it is the quality of this knowledge, however, which can be brought into question.
As I sit here, watching this computer screen, light in the form of photons is hitting my eyes and travelling as neural impulses to my brain. My brain then interprets these signals and creates for me a picture of my ever changing reality. The essential point here (and I'm aware of my incompitent knowledge of neurology) is that what I see is a representation of reality, created by me; it is not reality itself. The seems to have profound phenomenological implications, seeing that the world we accept as authentic is, in fact, our very own unique interpretation. If all we experience is our own, subjective recreation, then what exactly is "the world"? How can there be a confirmation of the existence of both objects and things in the external world if our method of verification involves an arguably biased re-creation?
Alterations in consciousness arise from abnormal fluctuations in certain neurotransmitter levels. There are four neurotransmitters responsible for all altered states of consciousness (acetylcholine, seretonin, dopamine and norepinephrine...I think!). The myriad studies undertaken to determine how various things affect changes in consciousness are essentially measuring the degrees to which humans vary their representations of the world. But is this just measuring the alterations of an already questionable reality? The foundations on which consciousness as it is conventionally accepted seem to shake lightly when it is seen from this vantage. Could altered states in fact bring us closer to a "real" reality, or do they take us further from it (whatever "it" is)?
The brain is the centre for interpretation in this sense. If it re-creates or reinterprets all external stimuli, then subjective reality, as a unique and individual phenomenon, falls into solipsism. All that we can know comes from our brains' calculations, which are assumably very subjective and thus unique, and which are approximations as opposed to exact empirical measurements. Essentially, it could be argued, each individual experiences a vastly different reality, which is based not on a certain universal reality we all partake in, but an interpretation of a reality which is unable to be grasped by human beings.
This perhaps brings into question the veritability of scientific/empirical findings. It also has profound existential effects; it leaves the human in his/her lonely existence, unable to verify others, or to know that what they see is the same as what others see. Sounds, images, sensations: all such stimuli are naturally refined in the process of interpretation, leaving the subject with knowledge only of their own, subjective phenomena, not of what truly is. A bundle of external data, garbled into what we call consciousness.
And implications thus arise, as previously mentioned, regarding altered states of consciousness. These altered states include sleep, hypnagogy (period between wakefulness and sleep), hypnapompy (period between sleep and wakefulness), drug-induced altered states as well as those that result from mental illness. It has been explained (See Hobson, The Dream Drugstore) how all of these states essentially work via the four neurotransmitters mentioned earlier. The brain chemicals associated with REM sleep, which is the primary dreaming state, have also been shown to be associated with other altered states, such as those induced by certain hallucinogens; and dopamine is linked with both mental illnesses such as Schizophrenia, as well as marijuana and other drug usage. Though my knowledge of these associations is limited, many other links have been found. It is extremely important to note that the quality of the fluctuations in consciousness is dependent not only on the links mentioned; many, many variables affect exactly how one's consciousness is affected and changed (for example, marijuana use obviously does not reproduce the experience of a schizophrenic).
So, what does this imply? Not only is solipsism an intriguing and plausible notion, but through power and authority humans are, in a way, prohibiting and promoting certain kinds of realities, none of which we can claim to be authentic (seeing as there is no way of knowing what an authentic reality is). By allowing prohibitions of consciousness-altering drugs, while advocating other drugs which have relatively similar effects but which have been deemed therapeutic in some way, people are being guided in how they interpret the world. In both cases, the same chemicals are being affected, just to different degrees. It could even be daringly said that convention, social mores, etc. are another method whereby what we select to interpret to create our world is being controlled by an external agent. Foucault was one of the first to see the conceptions of things like modern health and sexuality as ways of controlling the masses (i.e. through institutions); was this observation, perhaps, a way to modulate human consciousness and thus peoples' experience of reality?
This argument is not extensive, but it helps put into perspective the human accounts of reality. Orwellian thought-control becomes a very real thing when consciousness is seen as controlled by authorities, who could very well have sinister motives for allowing or disallowing substances. Perhaps this is the way of exploiting a ubiquitous unstable reality, one in which all we have is our own conceptions, our own calculations and selective re-creations of things. Existentially, solipsism leaves us isolated and alone - what if we could attain some form of collective consciousness through organic hallucinogens or other means? Is this being prohibited so as we can serve as the means to an end for those in power? The very comment seems so far-fetched, so counter-intuitive that the average reader will turn away, perhaps even laugh, at the prospect of drug promotion; this, when it is considered that plants have been an integral part of human communities for thousands fo years, simply demonstrates just how conditioned peoples' thoughts are in the modern era. But, to move on to a conclusion: the networks of thought and ideas this topic creates are endless and leave us baffled and alone in a world of our own creation; it does, however, raise important questions, which I think more people should address. To do this, humans en masse need to look past the universal social mores they have clung to for so long, and perhaps ask themselves what is really absurd in this existence.
T. Mautner, Dictionary of Philosophy, Penguin, 2005
A. Hobson, The Dream Drugstore: chemically altered states of consciousness, The MIT Press, Cambridge, Massachusetts, 2001