Horror vacui (nature's fear of vacuum) was expression related to the belief of Aristotle's followers denying the possible existence of empty space in nature on the grounds that where there is nothing, space cannot be defined. Wherever a vacuum would be on the threshold to develop, the nature would not allow for it and took its place immediately. This philosofical concept stems from Aristotle's book Physica where he maintained that the nature consisted of water, earth, air and fire. The lightest element, fire, would be directed upwards whereas the heaviest element, earth, downwards. On the other hand, the vacuum would not allow for any definition of "up" and "down" and therefore nature must abhor it.
- Karl Jousten et al. (2008). "The History Of Vacuum Science and Vacuum Technology", Handbook of Vacuum Technology. John Wiley & Sons, 2. ISBN 978-3-527-40723-1.