Inphilosophy and earlyphysics,horror vacui (Latin:horror of the vacuum) orplenism (/ˈpliːnɪzəm/)—commonly stated as "nature abhors a vacuum", for example bySpinoza^[1]—is a hypothesis attributed toAristotle, later criticized by theatomism ofEpicurus andLucretius, that nature contains novacuums because the denser surrounding material continuum would immediately fill the rarity of an incipient void.^[2]

Aristotle also argued against the void in a more abstract sense: since a void is merelynothingness, following his teacherPlato, nothingness cannot rightly be said to exist.^{[citation needed]} Furthermore, insofar as a void would be featureless, it could neither be encountered by the senses nor could its supposition lend additional explanatory power.Hero of Alexandria challenged the theory in the first century AD, but his attempts to create an artificial vacuum failed.^[3] The theory was debated in the context of 17th-centuryfluid mechanics, byThomas Hobbes andRobert Boyle,^[4] among others, and through the early 18th century bySir Isaac Newton andGottfried Leibniz.^[5][6]

As advanced by Aristotle inPhysics:

In a void, no one could say why a thing once set in motion should stop anywhere; for why should it stop here rather than here? So that a thing will either be at rest or must be movedad infinitum, unless something more powerful gets in its way.

Further, things are now thought to move into the void because it yields; but in a void this quality is present equally everywhere, so that things should move in all directions.

Further, the truth of what we assert is plain from the following considerations. We see the same weight or body moving faster than another for two reasons, either because there is a difference in what it moves through, as between water, air, and earth, or because, other things being equal, the moving body differs from the other owing to excess of weight or of lightness.

Now the medium causes a difference because it impedes the moving thing, most of all if it is moving in the opposite direction, but in a secondary degree even if it is at rest; and especially a medium that is not easily divided, i.e. a medium that is somewhat dense.A, then, will move throughB in timeG, and throughD, which is thinner, in timeE (if the length ofB is equal toD), in proportion to the density of the hindering body. For letB be water andD air; then by so much as air is thinner and more incorporeal than water,A will move throughD faster than throughB. Let the speed have the same ratio to the speed, then, that air has to water. Then if air is twice as thin, the body will traverseB in twice the time that it doesD, and the timeG will be twice the timeE. And always, by so much as the medium is more incorporeal and less resistant and more easily divided, the faster will be the movement.

Now there is no ratio in which the void is exceeded by body, as there is no ratio of 0 to a number. For if 4 exceeds 3 by 1, and 2 by more than 1, and 1 by still more than it exceeds 2, still there is no ratio by which it exceeds 0; for that which exceeds must be divisible into the excess + that which is exceeded, so that will be what it exceeds 0 by + 0. For this reason, too, a line does not exceed a point unless it is composed of points! Similarly the void can bear no ratio to the full, and therefore neither can movement through the one to movement through the other, but if a thing moves through the thickest medium such and such a distance in such and such a time, it moves through the void with a speed beyond any ratio. For letZ be void, equal in magnitude toB and toD. Then ifA is to traverse and move through it in a certain time,H, a time less thanE, however, the void will bear this ratio to the full. But in a time equal toH,A will traverse the partO ofA. And it will surely also traverse in that time any substanceZ which exceeds air in thickness in the ratio which the timeE bears to the timeH. For if the bodyZ be as much thinner thanD asE exceedsH,A, if it moves throughZ, will traverse it in a time inverse to the speed of the movement, i.e. in a time equal toH. If, then, there is no body inZ,A will traverseZ still more quickly. But we supposed that its traverse ofZ whenZ was void occupied the timeH. So that it will traverseZ in an equal time whetherZ be full or void. But this is impossible. It is plain, then, that if there is a time in which it will move through any part of the void, this impossible result will follow: it will be found to traverse a certain distance, whether this be full or void, in an equal time; for there will be some body which is in the same ratio to the other body as the time is to the time.^[2]

Plenism means "fullness", from Latinplēnum, English "plenty", cognate via Proto-Indo-European to "full". InAncient Greek, the term for the void is τὸ κενόν (to kenón).

The idea was restated as"Natura abhorret vacuum" byFrançois Rabelais in his series of books titledGargantua and Pantagruel in the 1530s.^[7] The theory was supported and restated byGalileo Galilei in the early 17th century as"Resistenza del vacuo". Galileo was surprised by the fact that water could not rise above a certain level in an aspiration tube in hissuction pump, leading him to conclude that there is a limit to the phenomenon.^[8]René Descartes proposed aplenic interpretation of atomism to eliminate the void, which he considered incompatible with his concept of space.^[5] The theory was rejected by later scientists, such as Galileo's pupilEvangelista Torricelli, who repeated his experiment withmercury.Blaise Pascal successfully repeated Galileo's and Torricelli's experiment and foresaw no reason why a perfect vacuum could not be achieved in principle.^[9] Scottish philosopherThomas Carlyle mentioned Pascal's experiment in theEdinburgh Encyclopædia in an 1823 article titled "Pascal".^[10]

• Obsolete theories in physics
• Vacuum

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