In mathematical logic, in particular in model theory and non-standard analysis, an internal set is a set that is a member of a model.
The concept of internal sets is a tool in formulating the transfer principle, which concerns the logical relation between the properties of the real numbers R, and the properties of a larger field denoted *R called the hyperreal numbers. The field *R includes, in particular, infinitesimal ("infinitely small") numbers, providing a rigorous mathematical justification for their use. Roughly speaking, the idea is to express analysis over R in a suitable language of mathematical logic, and then point out that this language applies equally well to *R. This turns out to be possible because at the set-theoretic level, the propositions in such a language are interpreted to apply only to internal sets rather than to all sets (note that the term "language" is used in a loose sense in the above).
Edward Nelson's internal set theory is not a constructivist version of non-standard analysis (but see Palmgren at constructive non-standard analysis). Its name should not mislead the reader: conventional infinitary accounts of non-standard analysis also use the concept of internal sets.
Read more about Internal Set: Internal Sets in The Ultrapower Construction, Internal Subsets of The Reals
Famous quotes containing the words internal and/or set:
“We all run on two clocks. One is the outside clock, which ticks away our decades and brings us ceaselessly to the dry season. The other is the inside clock, where you are your own timekeeper and determine your own chronology, your own internal weather and your own rate of living. Sometimes the inner clock runs itself out long before the outer one, and you see a dead man going through the motions of living.”
—Max Lerner (b. 1902)
“When you have come into the land that the LORD your God is giving you, and have taken possession of it and settled in it, and you say, “I will set a king over me, like all the nations that are around me,” you may indeed set over you a king whom the LORD your God will choose. One of your own community you may set as king over you; you are not permitted to put a foreigner over you, who is not of your own community.”
—Bible: Hebrew, Deuteronomy 17:14,15.