In science, an effective theory is a scientific theory which proposes to describe a certain set of observations, but explicitly without the claim or implication that the mechanism used by the theory has a direct counterpart in the actual causes of the observed phenomena to which the theory is fitted. That means that the theory proposes to model a certain effect, without proposing to model adequately any of the causes which contribute to the effect.

An early example is Newton's law of universal gravitation. Isaac Newton postulated his inverse-square law without further speculation, saying

I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. In this philosophy particular propositions are inferred from the phenomena, and afterwards rendered general by induction.^[1]

this hypothesis without further reasoning is referred as Newton's hypotheses non fingo.

For example, effective field theory is a method used to describe physical theories when there is a hierarchy of scales. Effective field theories in physics can include quantum field theories in which the fields are treated as fundamental, and effective theories describing phenomena in solid-state physics. For instance, the BCS theory of superconduction treats vibrations of the solid-state lattice as a "field" (i.e. without claiming that there is really a field), with its own field quanta, known as phonons. Such "effective particles" derived from effective fields are also known as quasiparticles.^{[citation needed]}

In a certain sense, quantum field theory, and any other currently known physical theory, could be described as "effective", as in being the "low energy limit" of an as-yet unknown theory of everything.^[2]