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verifiability principle

Criterion of meaningfulness associated with logical positivism and the Vienna Circle. Moritz Schlick's formulation “The meaning of a [declarative sentence] is the method of its verification” was close to the view held in pragmatism, and later in operationalism, that an assertion has factual meaning only if there is a difference in principle, open to test by observation, between the affirmation and the denial of the assertion. Thus, the statements of ethics, metaphysics, religion, and aesthetics were held to be meaningless. The verifiability criterion of meaningfulness was in part inspired by Albert Einstein's abandonment of the ether hypothesis and the notion of absolute simultaneity.

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or Heisenberg uncertainty principle or indeterminacy principle

Principle that states that the position and velocity of an object cannot both be measured exactly at the same time, and that the concepts of exact position and exact velocity together have no meaning in nature. Articulated by Werner Heisenberg in 1927, it applies only at the small scales of atoms and subatomic particles and is not noticeable for macroscopic objects, such as moving vehicles. Any attempt to measure the velocity of a subatomic particle precisely will displace the particle in an unpredictable way, thus invalidating any simultaneous measurement of its position. This displacement is a result of the wave nature of particles (see wave-particle duality). The principle also applies to other related pairs of variables, such as energy and time.

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Assertion proposed by Wolfgang Pauli that no two electrons in an atom can be in the same state or configuration at the same time. It accounts for the observed patterns of light emission from atoms. The principle has since been generalized to include the whole class of particles called fermions. The spin of such particles is always an odd whole-number multiple of 12. For example, electrons have spin 12, and can occupy two distinct states with opposite spin directions. The Pauli exclusion principle indicates, therefore, that only two electrons are allowed in each atomic energy state, leading to the successive buildup of orbitals around the nucleus. This prevents matter from collapsing to an extremely dense state.

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Fundamental principle of physics that in its weak form states that gravitational (see gravitation) and inertial (see inertia) masses are the same. Albert Einstein's stronger version states that gravitation and acceleration are indistinguishable. It implies that the effect of gravity is removed in a suitably accelerated reference frame, such as an elevator with its cable cut, in which a person would experience free fall.

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Hypothesis that the inertial forces acting on a body in accelerated motion are determined by the quantity and distribution of matter in the universe. Albert Einstein found its suggested connection between geometry and matter helpful in formulating his theory of general relativity; unaware that George Berkeley had proposed similar views in the 18th century, he attributed the idea to Ernst Mach. He abandoned the principle when he realized that inertia is assumed in the geodesic equation of motion (see geodesy) and need not depend on the existence of matter elsewhere in the universe.

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or Bernoulli's theorem

Principle that relates pressure, velocity, and height for a nonviscous fluid with steady flow. A consequence is that, for horizontal flow, as the speed of a fluid increases, the pressure it exerts decreases. Derived by Daniel Bernoulli (see Bernoulli family), the principle explains the lift of an airplane in motion. As the speed of the plane increases, air flows faster over the curved top of the wing than underneath. The upward pressure exerted by the air under the wing is thus greater than the pressure exerted downward above the wing, resulting in a net upward force, or lift. Race cars use the principle to keep their wheels pressed to the ground as they accelerate. A race car's spoiler—shaped like an upside-down wing, with the curved surface at the bottom—produces a net downward force.

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Law of buoyancy, discovered by Archimedes, which states that any object that is completely or partially submerged in a fluid at rest is acted on by an upward, or buoyant, force. The magnitude of this force is equal to the weight of the fluid displaced by the object. The volume of fluid displaced is equal to the volume of the portion of the object submerged.

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In political science, a principle-policy puzzle is a disconnect between support for a principle and support for a policy supporting that principle.

The primary example is the apparent inconsistency between American support of the principle of racial equality, and lack of support of a policy intended to achieve racial equality, especially among those most politically aware. The seeming paradox is explained by noting that lack of support for affirmative action comes from conservative ideology, such that opponents of government policies intended to realize racial equality are not necessarily hypocrites.


The principle-policy puzzle was described in Reasoning and Choice (1991) by Paul M. Sniderman, Richard A. Brody, and Philip E. Tetlock.

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