Subjectivism is the doctrine that "our own mental activity is the only unquestionable fact of our experience",  instead of shared or communal, and that there is no external or objective truth.
While Thomas Hobbes was an early proponent of subjectivism,   the success of this position is historically attributed to Descartes and his methodic doubt. He used it as an epistemological tool to prove the opposite (an objective world of facts independent of one's own knowledge, ergo the "Father of Modern Philosophy" inasmuch as his views underlie a scientific worldview).  Subjectivism accords primacy to subjective experience as fundamental of all measure and law.  In extreme forms like Solipsism, it may hold that the nature and existence of every object depends solely on someone's subjective awareness of it. One may consider the qualified empiricism of George Berkeley in this context, given his reliance on God as the prime mover of human perception.
Subjectivism is a label used to denote the philosophical tenet that "our own mental activity is the only unquestionable fact of our experience."  While Thomas Hobbes was an early proponent of subjectivism,   The success of this position is historically attributed to Descartes and his methodic doubt.  Subjectivism has historically been condemned by Christian theologians, which oppose to it the objective authority of the church, the Christian dogma, and the revealed truth of the Bible.   Christian theologians, and Karl Barth in particular, have also condemned anthropocentrism as a form of subjectivism.  
Metaphysical subjectivism is the theory that reality is what we perceive to be real, and that there is no underlying true reality that exists independently of perception. One can also hold that it is consciousness rather than perception that is reality (idealism). This is in contrast to metaphysical objectivism and philosophical realism, which assert that there is an underlying 'objective' reality which is perceived in different ways.
This viewpoint should not be confused with the stance that "all is illusion" or that "there is no such thing as reality." Metaphysical subjectivists hold that reality is real enough. They conceive, however, that the nature of reality as related to a given consciousness is dependent on that consciousness. This has its philosophical basis in the writings of Descartes (see cogito ergo sum ), and forms a cornerstone of Søren Kierkegaard's philosophy.
Recently, more modest versions of metaphysical subjectivism have been explored. For example, I might hold that it is a fact that chocolate is tasty, even though I recognize that it is not tasty to everyone. This would imply that there are facts that are subjective. (Analogously, one might hold that it is a fact that it is winter in the Northern Hemisphere, even though this is not always the case, implying that some facts are temporary.) Giovanni Merlo has developed a specific version of metaphysical subjectivism, under which subjective facts always concern mental properties.  With Giulia Pravato, he has argued that his version of subjectivism provides a natural way to be both a realist and a relativist about, for example, the proposition that chocolate is tasty -- it is part of reality (a subjective fact) that chocolate is tasty, but that doesn't mean it's necessarily true from another's point of view.  Caspar Hare's theory of egocentric presentism is another, closely related example.
One possible extension of subjectivist thought is that conscious experience is available to all objectively perceivable substrates. Upon viewing images produced by a camera on the rocking side of an erupting volcano, one might suppose that their relative motion followed from a subjective conscious within the volcano. These properties might also be attributed to the camera or its various components as well.
In this way, though, subjectivism morphs into a related doctrine, panpsychism, the belief that every objective entity (or event) has an inward or subjective aspect.
Ethical subjectivism is the meta-ethical belief that ethical sentences reduce to factual statements about the attitudes and/or conventions of individual people, or that any ethical sentence implies an attitude held by someone. As such, it is a form of moral relativism in which the truth of moral claims is relative to the attitudes of individuals  (as opposed to, for instance, communities). Consider the case this way — to a person imagining what it's like to be a cat, catching and eating mice is perfectly natural and morally sound. To a person imagining they are a mouse, being hunted by cats is morally abhorrent. Though this is a loose metaphor, it serves to illustrate the view that each individual subject has their own understanding of right and wrong.
An ethical subjectivist might propose, for example, that what it means for something to be morally right is just for it to be approved of. (This can lead to the belief that different things are right according to each idiosyncratic moral outlook.) One implication of these beliefs is that, unlike the moral skeptic or the non-cognitivist, the subjectivist thinks that ethical sentences, while subjective, are nonetheless the kind of thing that can be true or false depending on situation.
Broadly speaking, there are two views on Bayesian probability that interpret the probability concept in different ways. In probability, a subjectivist stand is the belief that probabilities are simply degrees-of-belief by rational agents in a certain proposition, and which have no objective reality in and of themselves. According to the subjectivist view, probability measures a "personal belief".  For this kind of subjectivist, a phrase having to do with probability simply asserts the degree to which the subjective actor believes their assertion is true or false. As a consequence, a subjectivist has no problem with differing people giving different probabilities to an uncertain proposition, and all being correct.
Many modern machine learning methods are based on objectivist Bayesian principles.  According to the objectivist view, the rules of Bayesian statistics can be justified by requirements of rationality and consistency and interpreted as an extension of logic.   In attempting to justify subjective probability, Bruno de Finetti created the notion of philosophical coherence. According to his theory, a probability assertion is akin to a bet, and a bet is coherent only if it does not expose the wagerer to loss if their opponent chooses wisely. To explain his meaning, de Finetti created a thought-experiment to illustrate the need for principles of coherency in making a probabilistic statement. In his scenario, when someone states their degree-of-belief in something, one places a small bet for or against that belief and specifies the odds, with the understanding that the other party to the bet may then decide which side of the bet to take. Thus, if Bob specifies 3-to-1 odds against a proposition A, his opponent Joe may then choose whether to require Bob to risk $1 in order to win $3 if proposition A is found to be true, or to require Bob to risk $3 in order to win $1 if the proposition A is not true. In this case, it is possible for Joe to win over Bob. According to de Finetti, then, this case is incoherent. 
In analytic philosophy, anti-realism is a position which encompasses many varieties such as metaphysical, mathematical, semantic, scientific, moral and epistemic. The term was first articulated by British philosopher Michael Dummett in an argument against a form of realism Dummett saw as 'colorless reductionism'.
Bayesian probability is an interpretation of the concept of probability, in which, instead of frequency or propensity of some phenomenon, probability is interpreted as reasonable expectation representing a state of knowledge or as quantification of a personal belief.
Epistemology, or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics.
In metaphilosophy and ethics, meta-ethics is the study of the nature, scope, and meaning of moral judgment. It is one of the three branches of ethics generally studied by philosophers, the others being normative ethics and applied ethics.
Relativism is a family of philosophical views which deny claims to objectivity within a particular domain and assert that valuations in that domain are relative to the perspective of an observer or the context in which they are assessed. There are many different forms of relativism, with a great deal of variation in scope and differing degrees of controversy among them. Moral relativism encompasses the differences in moral judgments among people and cultures. Epistemic relativism holds that there are no absolute principles regarding normative belief, justification, or rationality, and that there are only relative ones. Alethic relativism is the doctrine that there are no absolute truths, i.e., that truth is always relative to some particular frame of reference, such as a language or a culture. Some forms of relativism also bear a resemblance to philosophical skepticism. Descriptive relativism seeks to describe the differences among cultures and people without evaluation, while normative relativism evaluates the morality or truthfulness of views within a given framework.
Moral relativism or ethical relativism is used to describe several philosophical positions concerned with the differences in moral judgments across different peoples and cultures. An advocate of such ideas is often referred to as a relativist for short.
Moral realism is the position that ethical sentences express propositions that refer to objective features of the world, some of which may be true to the extent that they report those features accurately. This makes moral realism a non-nihilist form of ethical cognitivism with an ontological orientation, standing in opposition to all forms of moral anti-realism and moral skepticism, including ethical subjectivism, error theory ; and non-cognitivism. Within moral realism, the two main subdivisions are ethical naturalism and ethical non-naturalism.
Bruno de Finetti was an Italian probabilist statistician and actuary, noted for the "operational subjective" conception of probability. The classic exposition of his distinctive theory is the 1937 "La prévision: ses lois logiques, ses sources subjectives," which discussed probability founded on the coherence of betting odds and the consequences of exchangeability.
Subjective may refer to:
Cognitivism is the meta-ethical view that ethical sentences express propositions and can therefore be true or false, which noncognitivists deny. Cognitivism is so broad a thesis that it encompasses moral realism, ethical subjectivism, and error theory.
Ethical subjectivism or moral non-objectivism is the meta-ethical view which claims that:
Subjectivity in a philosophical context has to do with a lack of objective reality. Subjectivity has been given various and ambiguous definitions by differing sources as it is not often the focal point of philosophical discourse. However, it is related to ideas of consciousness, agency, personhood, philosophy of mind, reality, and truth. Three common definitions include that subjectivity is the quality or condition of:
In gambling, a Dutch book or lock is a set of odds and bets, established by the bookmaker, that ensures that the bookmaker will profit—at the expense of the gamblers—regardless of the outcome of the event on which the gamblers bet. It is associated with probabilities implied by the odds not being coherent.
Moral nihilism is the meta-ethical view that nothing is morally right or wrong.
This glossary of philosophy is a list of definitions of terms and concepts relevant to philosophy and related disciplines, including logic, ethics, and theology.
Projectivism in philosophy involves attributing (projecting) qualities to an object as if those qualities actually belong to it. It is a theory for how people interact with the world and has been applied in both ethics and general philosophy. It is derived from the Humean idea that all judgements about the world derive from internal experience, and that we therefore project our emotional state onto the world and interpret it through the lens of our own experience. Projectivism has proved to be a problem for moral realists, who assert that moral judgements can be determined from empirical facts, i.e., some things are objectively right or wrong.
Ideal observer theory is the meta-ethical view which claims that ethical sentences express truth-apt propositions about the attitudes of a hypothetical ideal observer. In other words, ideal observer theory states that ethical judgments should be interpreted as statements about the judgments that a neutral and fully informed observer would make; "x is good" means "an ideal observer would approve of x".
The main idea [of the ideal observer theory] is that ethical terms should be defined after the pattern of the following example: "x is better than y" means "If anyone were, in respect of x and y, fully informed and vividly imaginative, impartial, in a calm frame of mind and otherwise normal, he would prefer x to y.
In philosophy, objectivity is the concept of truth independent from individual subjectivity. A proposition is considered to have objective truth when its truth conditions are met without bias caused by the mind of a sentient being. Scientific objectivity refers to the ability to judge without partiality or external influence. Objectivity in the moral framework calls for moral codes to be assessed based on the well-being of the people in the society that follow it. Moral objectivity also calls for moral codes to be compared to one another through a set of universal facts and not through subjectivity.
Egocentric presentism is a form of solipsism introduced by Caspar Hare in which other persons can be conscious, but their experiences are simply not present.
Bayesian epistemology is a formal approach to various topics in epistemology that has its roots in Thomas Bayes' work in the field of probability theory. One advantage of its formal method in contrast to traditional epistemology is that its concepts and theorems can be defined with a high degree of precision. It is based on the idea that beliefs can be interpreted as subjective probabilities. As such, they are subject to the laws of probability theory, which act as the norms of rationality. These norms can be divided into static constraints, governing the rationality of beliefs at any moment, and dynamic constraints, governing how rational agents should change their beliefs upon receiving new evidence. The most characteristic Bayesian expression of these principles is found in the form of Dutch books, which illustrate irrationality in agents through a series of bets that lead to a loss for the agent no matter which of the probabilistic events occurs. Bayesians have applied these fundamental principles to various epistemological topics but Bayesianism does not cover all topics of traditional epistemology. The problem of confirmation in the philosophy of science, for example, can be approached through the Bayesian principle of conditionalization by holding that a piece of evidence confirms a theory if it raises the likelihood that this theory is true. Various proposals have been made to define the concept of coherence in terms of probability, usually in the sense that two propositions cohere if the probability of their conjunction is higher than if they were neutrally related to each other. The Bayesian approach has also been fruitful in the field of social epistemology, for example, concerning the problem of testimony or the problem of group belief. Bayesianism still faces various theoretical objections that have not been fully solved.