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In their most common sense, the terms thought and thinking refer to cognitive processes that can happen independently of sensory stimulation. Their most paradigmatic forms are judging, reasoning, concept formation, problem solving, and deliberation. But other mental processes, like considering an idea, memory, or imagination, are also often included. These processes can happen internally independent of the sensory organs, unlike perception. But when understood in the widest sense, any mental event may be understood as a form of thinking, including perception and unconscious mental processes. In a slightly different sense, the term thought refers not to the mental processes themselves but to mental states or systems of ideas brought about by these processes.

Various theories of thinking have been proposed, some of which aim to capture the characteristic features of thought. Platonists hold that thinking consists in discerning and inspecting Platonic forms and their interrelations. It involves the ability to discriminate between the pure Platonic forms themselves and the mere imitations found in the sensory world. According to Aristotelianism, to think about something is to instantiate in one's mind the universal essence of the object of thought. These universals are abstracted from sense experience and are not understood as existing in a changeless intelligible world, in contrast to Platonism. Conceptualism is closely related to Aristotelianism: it identifies thinking with mentally evoking concepts instead of instantiating essences. Inner speech theories claim that thinking is a form of inner speech in which words are silently expressed in the thinker's mind. According to some accounts, this happens in a regular language, like English or French. The language of thought hypothesis, on the other hand, holds that this happens in the medium of a unique mental language called Mentalese. Central to this idea is that linguistic representational systems are built up from atomic and compound representations and that this structure is also found in thought. Associationists understand thinking as the succession of ideas or images. They are particularly interested in the laws of association that govern how the train of thought unfolds. Behaviorists, by contrast, identify thinking with behavioral dispositions to engage in public intelligent behavior as a reaction to particular external stimuli. Computationalism is the most recent of these theories. It sees thinking in analogy to how computers work in terms of the storage, transmission, and processing of information.

Various types of thinking are discussed in the academic literature. A judgment is a mental operation in which a proposition is evoked and then either affirmed or denied. Reasoning, on the other hand, is the process of drawing conclusions from premises or evidence. Both judging and reasoning depend on the possession of the relevant concepts, which are acquired in the process of concept formation. In the case of problem solving, thinking aims at reaching a predefined goal by overcoming certain obstacles. Deliberation is an important form of practical thought that consists in formulating possible courses of action and assessing the reasons for and against them. This may lead to a decision by choosing the most favorable option. Both episodic memory and imagination present objects and situations internally, in an attempt to accurately reproduce what was previously experienced or as a free rearrangement, respectively. Unconscious thought is thought that happens without being directly experienced. It is sometimes posited to explain how difficult problems are solved in cases where no conscious thought was employed.

Thought is discussed in various academic disciplines. Phenomenology is interested in the experience of thinking. An important question in this field concerns the experiential character of thinking and to what extent this character can be explained in terms of sensory experience. Metaphysics is, among other things, interested in the relation between mind and matter. This concerns the question of how thinking can fit into the material world as described by the natural sciences. Cognitive psychology aims to understand thought as a form of information processing. Developmental psychology, on the other hand, investigates the development of thought from birth to maturity and asks which factors this development depends on. Psychoanalysis emphasizes the role of the unconscious in mental life. Other fields concerned with thought include linguistics, neuroscience, artificial intelligence, biology, and sociology. Various concepts and theories are closely related to the topic of thought. The term " law of thought " refers to three fundamental laws of logic: the law of contradiction, the law of excluded middle, and the principle of identity. Counterfactual thinking involves mental representations of non-actual situations and events in which the thinker tries to assess what would be the case if things had been different. Thought experiments often employ counterfactual thinking in order to illustrate theories or to test their plausibility. Critical thinking is a form of thinking that is reasonable, reflective, and focused on determining what to believe or how to act. Positive thinking involves focusing one's attention on the positive aspects of one's situation and is intimately related to optimism.