In their most common sense, thought and thinking refer to cognitive processes that occur independently of direct sensory stimulation. Core forms include judging, reasoning, concept formation, problem solving, and deliberation. Other processes, such as entertaining an idea, memory, or imagination, are also frequently considered types of thought. Unlike perception, these activities can occur without immediate input from the sensory organs. In a broader sense, any mental event—including perception and unconscious processes—may be described as a form of thought. The term can also denote not the process itself, but the resulting mental states or systems of ideas.

A variety of theories attempt to explain the nature of thinking. Platonism holds that thought involves discerning eternal forms and their interrelations, distinguishing these pure entities from their imperfect sensory imitations. Aristotelianism interprets thinking as instantiating the universal essence of an object within the mind, derived from sense experience rather than a changeless realm. Conceptualism, closely related to Aristotelianism, identifies thinking with the mental evocation of concepts. Inner speech theories suggest that thought takes the form of silent verbal expression, sometimes in a natural language and sometimes in a specialized "mental language," or Mentalese, as proposed by the language of thought hypothesis. Associationism views thought as the succession of ideas governed by laws of association, while behaviorism reduces thinking to behavioral dispositions that generate intelligent actions in response to stimuli. More recently, computationalism compares thought to information processing, storage, and transmission in computers.

Different types of thinking are recognized in philosophy and psychology. Judgement involves affirming or denying a proposition; reasoning draws conclusions from premises or evidence. Both depend on concepts acquired through concept formation. Problem solving aims at achieving specific goals by overcoming obstacles, while deliberation evaluates possible courses of action before selecting one. Episodic memory and imagination internally represent objects or events, either as faithful reproductions or novel rearrangements. Unconscious thought refers to mental activity that occurs without conscious awareness and is sometimes invoked to explain solutions reached without deliberate effort.

The study of thought spans many disciplines. Phenomenology examines the subjective experience of thinking, while metaphysics addresses how mental processes relate to matter in a naturalistic framework. Cognitive psychology treats thought as information processing, whereas developmental psychology explores its growth from infancy to adulthood. Psychoanalysis emphasizes unconscious processes, and fields such as linguistics, neuroscience, artificial intelligence, biology, and sociology also investigate different aspects of thought. Related concepts include the classical laws of thought (identity, non-contradiction, excluded middle), counterfactual thinking (imagining alternatives to reality), thought experiments (testing theories through hypothetical scenarios), critical thinking (reflective evaluation of beliefs and actions), and positive thinking (focusing on beneficial aspects of situations, often linked to optimism).

The terms "thought" and "thinking" are used in different ways in psychology and philosophy. In their most common sense, they refer to conscious processes that occur independently of direct sensory input. This includes activities such as considering an idea, evaluating a preposition, or making a judgement. In this sense, memory and imagination count as forms of thought, while perception does not. In a narrower sense, only the most typical cases are called thought-specifically conscious, conceptual or linguistic processes such as judging, inferring, problem-solving, and deliberating. Sometimes, however, the terms are understood in a much broader sense to include all mental processes, conscious or unconscious. In this wide usage, they can be treated as synonymous with mind, as in the Cartesian tradition (where the mind is described as a "thinking thing") and in the cognitive sciences. Some accounts further add that only processes leading to intelligent behavior should count as thought. A common contrast in the literature is drawn between thinking and feeling. In this distinction, thinking is seen as a rational, dispassionate activity, while feeling involves direct emotional engagement.

The words thought and thinking can also refer to the results of these processes, such as beliefs, mental states, or systems of ideas held by an individual or shared within a group. Academic discussions often leave implicit which of these senses is intended.

The word thought derives from Old English þoht or geþoht, from the stem of þencan ("to conceive in the mind, consider").

Many different theories of thinking have been developed. They attempt to describe the main features and processes involved in thinking. These theories are not necessarily mutually exclusive, meaning that some of them can be combined without contradiction.