Fluxion

Newton's introduction of the notions "fluent" and "fluxion" in his 1736 book

A fluxion is the instantaneous rate of change, or gradient, of a fluent (a time-varying quantity, or function) at a given point. Fluxions were introduced by Isaac Newton to describe his form of a time derivative (a derivative with respect to time). Newton introduced the concept in 1665 and detailed them in his mathematical treatise, Method of Fluxions. Fluxions and fluents made up Newton's early calculus.

Example

If the fluent Template:Tmath is defined as $y=t^2$ (where Template:Tmath is time) the fluxion (derivative) at $t=2$ is:

$\dot{y}=\frac{\mathrm{\Delta }y}{\mathrm{\Delta }t}=\frac{(2+o{)}^2-2^2}{(2+o)-2}=\frac{4+4o+o^2-4}{2+o-2}=\frac{4o+o^2}{o}$

Here Template:Tmath is an infinitely small amount of time. So, the term Template:Tmath is second order infinite small term and according to Newton, we can now ignore Template:Tmath because of its second order infinite smallness comparing to first order infinite smallness of Template:Tmath. So, the final equation gets the form:

$\dot{y}=\frac{\mathrm{\Delta }y}{\mathrm{\Delta }t}=\frac{4o}{o}=4$

He justified the use of Template:Tmath as a non-zero quantity by stating that fluxions were a consequence of movement by an object.

Criticism

Bishop George Berkeley, a prominent philosopher of the time, denounced Newton's fluxions in his essay The Analyst, published in 1734. Berkeley refused to believe that they were accurate because of the use of the infinitesimal Template:Tmath. He did not believe it could be ignored and pointed out that if it was zero, the consequence would be division by zero. Berkeley referred to them as "ghosts of departed quantities", a statement which unnerved mathematicians of the time and led to the eventual disuse of infinitesimals in calculus.

Towards the end of his life Newton revised his interpretation of Template:Tmath as infinitely small, preferring to define it as approaching zero, using a similar definition to the concept of limit. He believed this put fluxions back on safe ground. By this time, Leibniz's derivative (and his notation) had largely replaced Newton's fluxions and fluents, and remains in use today.

See also

• History of calculus
• Newton's notation
• Hyperreal number: A modern formalization of the reals that includes infinity and infinitesimals
• Nonstandard analysis

References

Further reading

• Template:Cite book