the early 20th century:

Brouwer reduced the debate to the use of proofs designed from "negative" or "non-existence" versus "constructive" proof:
::According to Brouwer, a statement that an object exists having a given property means that, and is only proved, when a method is known which in principle at least will enable such an object to be found or constructed ...
::Hilbert naturally disagreed.
::"pure existence proofs have been the most important landmarks in the historical development of our science," he maintained.