1.  theory(n = noun.cognition) - a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena; "theories can incorporate facts and laws and tested hypotheses"; "true in fact and theory"
is a kind of explanation
has parts: law, law of nature, hypothesis, possibility, theory
has particulars: reductionism, blastogenesis, preformation, theory of preformation, scientific theory, field theory, economic theory, atomic theory, atomism, atomist theory, atomistic theory, holism, holistic theory, structural sociology, structuralism, structural anthropology, structuralism
Derived forms adjective theoretic₁, noun theoretician₁, noun theorist₁

2.  theory(n = noun.cognition) hypothesis, possibility - a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena; "a scientific hypothesis that survives experimental testing becomes a scientific theory"; "he proposed a fresh theory of alkalis that later was accepted in chemical practices"
is a kind of concept, conception, construct
is a part of theory
has particulars: hypothetical, gemmule, framework, model, theoretical account, conjecture, speculation, assumption, supposal, supposition, historicism
Derived forms adjective theoretic₁, noun theoretician₁, verb theorise₁, noun theorist₁, verb theorize₁

3.  theory(n = noun.cognition) - a belief that can guide behavior; "the architect has a theory that more is less"; "they killed him on the theory that dead men tell no tales"
is a kind of belief
has particulars: egoism, hodgepodge, jumble, patchwork
Derived forms verb theorise₁, noun theorist₁, verb theorize₁

1.  A doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation. [1913 Webster]
" “This word is employed by English writers in a very loose and improper sense. It is with them usually convertible into hypothesis, and hypothesis is commonly used as another term for conjecture. The terms theory and theoretical are properly used in opposition to the terms practice and practical. In this sense, they were exclusively employed by the ancients; and in this sense, they are almost exclusively employed by the Continental philosophers.” Sir W. Hamilton. " Sir W. Hamilton. [1913 Webster]

2.  An exposition of the general or abstract principles of any science; as, the theory of music. [1913 Webster]

3.  The science, as distinguished from the art; as, the theory and practice of medicine. [1913 Webster]

4.  The philosophical explanation of phenomena, either physical or moral; as, Lavoisier's theory of combustion; Adam Smith's theory of moral sentiments. [1913 Webster]

theory, n. (pl. -ies)
1 a supposition or system of ideas explaining something, esp. one based on general principles independent of the particular things to be explained (opp. HYPOTHESIS) (atomic theory; theory of evolution).
2 a speculative (esp. fanciful) view (one of my pet theories).
3 the sphere of abstract knowledge or speculative thought (this is all very well in theory, but how will it work in practice?).
4 the exposition of the principles of a science etc. (the theory of music).
5 Math. a collection of propositions to illustrate the principles of a subject (probability theory; theory of equations).

Etymology:
LL theoria f. Gk theoria f. theoros spectator f. theoreo look at

N  attribution, theory, etiology, ascription, reference to, rationale, accounting for, palaetiology, imputation, derivation from, filiation, affiliation, pedigree, explanation, reason why, attributed, attributable, referable to, referrible to, due to, derivable from, owing to, putative, ecbatic, hence, thence, therefore, for, since, on account of, because, owing to, on that account, from this cause, from that cause, thanks to, forasmuch as, whence, propter hoc, why? wherefore? whence? how comes it, how is it, how happens it? how does it happen? in some way, in some such way, somehow, somehow or other, that is why, hinc illae lachrymae.

N  idea, notion, conception, thought, apprehension, impression, perception, image, eidolon, sentiment, reflection, observation, consideration, abstract idea, archetype, formative notion, guiding conception, organizing conception, image in the mind, regulative principle, view, theory, conceit, fancy, phantasy, point of view, field of view.

N  knowledge, cognizance, cognition, cognoscence, acquaintance, experience, ken, privity, insight, familiarity, comprehension, apprehension, recognition, appreciation, intuition, conscience, consciousness, perception, precognition, acroamatics, light, enlightenment, glimpse, inkling, glimmer, glimmering, dawn, scent, suspicion, impression, discovery, system of knowledge, body of knowledge, science, philosophy, pansophy, acroama, theory, aetiology, etiology, circle of the sciences, pandect, doctrine, body of doctrine, cyclopedia, encyclopedia, school, tree of knowledge, republic of letters, erudition, learning, lore, scholarship, reading, letters, literature, book madness, book learning, bookishness, bibliomania, bibliolatry, information, general information, store of knowledge, education, culture, menticulture, attainments, acquirements, acquisitions, accomplishments, proficiency, practical knowledge, liberal education, dilettantism, rudiments, deep knowledge, profound knowledge, solid knowledge, accurate knowledge, acroatic knowledge, acroamatic knowledge, vast knowledge, extensive knowledge, encyclopedic knowledge, encyclopedic learning, omniscience, pantology, march of intellect, progress of science, advance of science, advance of learning, schoolmaster abroad, scholar, knowing, cognitive, acroamatic, aware of, cognizant of, conscious of, acquainted with, made acquainted with, privy to, no stranger to, au fait with, au courant, in the secret, up to, alive to, behind the scenes, behind the curtain, let into, apprized of, informed of, undeceived, proficient with, versed with, read with, forward with, strong with, at home in, conversant with, familiar with, erudite, instructed, leaned, lettered, educated, well conned, well informed, well read, well grounded, well educated, enlightened, shrewd, savant, blue, bookish, scholastic, solid, profound, deep-read, book- learned, accomplished, omniscient, self-taught, known, ascertained, well-known, recognized, received, notorious, noted, proverbial, familiar, familiar as household words, familiar to every schoolboy, hackneyed, trite, trivial, commonplace, cognoscible, cognizable, to one's knowledge, to the best of one's knowledge, one's eyes being opened, ompredre tout c'est tout pardonner, to know all is to pardon all, empta dolore docet experientia, gnothi seauton, half our knowledge we must snatch not take, Jahre lehren mehr als Bucher, years teach more than books, knowledge comes but wisdom lingers, knowledge is power, les affaires font les hommes, nec scire fas est omnia, the amassed thought and experience of innumerable, was ich nicht weiss macht mich nicht heiss.

N  supposition, assumption, assumed position, postulation, condition, presupposition, hypothesis, blue sky hypothesis, postulate, postulatum, theory, thesis, theorem, data, proposition, position, proposal, presumption, divination, conjecture, guess, guesswork, speculation, rough guess, shot, shot in the dark, conjecturality, surmise, suspicion, sneaking suspicion, estimate, approximation (nearness), inkling, suggestion, hint, intimation, notion, impression, bare supposition, vague supposition, loose supposition, loose suggestion, association of ideas, (analogy), metonym, metonymy, simile (metaphor), conceit, idea, thought, original idea, invention (imagination), supposing, given, mooted, postulatory, assumed, supposititious, suppositive, suppositious, gratuitous, speculative, conjectural, hypothetical, theoretical, academic, supposable, presumptive, putative, suppositional, suggestive, allusive, if, if so be, an, on the supposition, ex hypothesi, in the case, in the event of, quasi, as if, provided, perhaps, for aught one knows.

arrhenius theory of dissociation, association theory, atomic theory, atomist theory, atomistic theory, big bang theory, big-bang theory, bohr theory, cell theory, communication theory, control theory, corpuscular theory, corpuscular theory of light, domino theory, economic theory, einstein's general theory of relativity, einstein's special theory of relativity, einstein's theory of relativity, field theory, galois theory, game theory, general relativity theory, general theory of relativity, germ theory, grand unification theory, grand unified theory, gravitational theory, group theory, holistic theory, in theory, information theory, kinetic theory, kinetic theory of gases, kinetic theory of heat, malthusian theory, newton's theory of gravitation, ostwald's theory of indicators, philosophical theory, plate tectonic theory, political theory, probability theory, quantum field theory, quantum theory, relativity theory, scientific theory, set theory, side-chain theory, special relativity theory, special theory of relativity, string theory, theory of dissociation, theory of electrolytic dissociation, theory of evolution, theory of games, theory of gravitation, theory of gravity, theory of indicators, theory of inheritance, theory of organic evolution, theory of preformation, theory of punctuated equilibrium, theory of relativity, undulatory theory, vortex theory, wave theory, wave theory of light