Conversation so far:
Did Piet Mondrian paint at all in the Netherlands?
After a strict Protestant upbringing, in 1892, Mondrian entered the Academy for Fine Art in Amsterdam. He began his career as a teacher in primary education; he also practiced painting.
Are there any other interesting aspects about Mondrian's time in the Netherlands besides practicing painting there?
While Mondrian was visiting the Netherlands in 1914, World War I began, forcing him to remain there for the duration of the conflict.
Did Mondrian have any influences?
Piet Mondrian stayed at the Laren artists' colony, where he met Bart van der Leck and Theo van Doesburg, who were both undergoing their own personal journeys toward abstraction.
Did Mondrian collaborate with Doesburg?
With Van Doesburg, Mondrian founded De Stijl (The Style), a journal of the De Stijl Group, in which he first published essays defining his theory, which he called neoplasticism.
What is DeStijl?
The term De Stijl is used to refer to a body of work from 1917 to 1931 founded in the Netherlands. Proponents of De Stijl advocated pure abstraction and universality
Neoplasticism is what?
De Stijl, Dutch for The Style, also known as Neoplasticism, was a Dutch art movement founded in 1917 in Leiden. De Stijl consisted of artists and architects.

Question: How was theory defined?
Self-contained rewrite: How was neoplasticism defined?