Subjectivism begins with personal experience. One might actually regard philosophical subjectivism as doing philosophy from the inside-out (which can eventually lead to critical-realism/non-realism). Both René Descartes (1596-1650) and Immanuel Kant (1724-1804) attempted to construct philosophical systems from this starting point (although in the end both were realists). In the modern world subjectivist philosophies have become very popular as they challenge the notion of absolute Truth which allows people to democratize truths. This means truths become relative to each person. As a result, a society built on subjectivist principles is believed to be tolerant and willing to allow people to live and let live (providing they do not harm others – which, ironically, is not a subjective, and therefore relative, statement).
However, the fact that subjectivism is the popular voice does not mean it is the only voice. Aside from its attractive quality of democratizing truth subjectivism brings many problems in terms of epistemology. This is no more so than how one can attach any notion of permanence to an ever-changing array of individual perceptions. It was for this reason that Plato (428-347 BCE) believed our understanding of ‘reality’ had to start from the outside-in (realism). His philosophy may be said to challenge subjectivism in order to establish the existence of Ultimate Truth (The Forms) which he believed lay behind the ever-changing world we live in (and are the source of our perceptions within it). Of course, this raises the question of whether there is an Ultimate Truth and if so, who has it. In the Middle Ages the Catholic Church was regarded as the source and preserver of such truth. However, the atrocities committed on behalf of the Church during the Crusades (and the subsequent Inquisition and Conquistadors), for the cause of ‘truth’ led many to become suspicious of realist philosophies. Yet just because realist philosophies have been misused in the past this does not mean they cannot be useful once again in the future. Subjectivism may currently be the loudest voice but it is not the only voice.
The fact that so many people see value in the subjectivist starting point for philosophy must mean that in some way it works. Subjectivism certainly takes seriously the personal and cultural influences which affect the pursuit of objective knowledge. Such influences can often be over-looked by realists who claim to have access to Ultimate Truth. In Western societies the notion of religious truth (realism) has largely been overturned by the scientific enterprise (particularly the scientific methodology based on establishing truth through testing, observation and proof). As religions generally deal with things that are largely outside this world they have struggled to retain their place as purveyors of Ultimate Truth about the world we live in.