After the revolution in the basis of the Mechanics done by the uncertainty principle there was a general change, more or less conscious and sharp, in the philosophical opinions of the majority of the physicists.
The Laplacian determinism had supplied a strong support to a materialistic conception of the world, that is to the idea that exists an objective reality beyond the human feelings, that this reality is knowable and that its spatio-temporal evolution may be described in terms of cause and effect.
But after the birth of Quantum Mechanics several currents of thought based on the negation of such assumptions found a formidable "scientific" support: the materialistic conceptions are dogmatic, metaphysic, and harmful to the progress of the knowledge. These currents were mainly connected to the interpretation of Quantum Mechanics of the so-called 'Copenhagen School', formed by a group of physicists led by the Danish Niels Bohr, whose intellectual formation was deeply influenced by the existentialism of Søren Kierkegaard.
Simplifying a lot, we can try to expose the positions of the Copenhagen School with the following affirmations:
But many among the same physicists who had contributed as protagonists to the development of Quantum Mechanics, like Einstein, Plank, Schrödinger, De Broglie, while they had life, remained irreducible antagonists of the Copenhagen School, even if this became the predominant orthodoxy.
In particular, the methodological objections of Einstein, that his opponents would have defined ingenuous, were many times explicitly declared by him:
What does not satisfy me in that theory, from the standpoint of principle, is its attitude
towards that which appears to me to be the programmatic aim of all physics:
the complete description of any (individual) real situation
(as it supposedly exists irrespective of any act of observation or substantiation).
Whenever the positivistically inclined modern physicist hears such a formulation
his reaction is that of a pitying smile.
He says to himself: "there we have the naked formulation of a metaphysical prejudice,
empty of content, a prejudice,
moreover, the conquest of which constitutes the major epistemological achievement of physicists
within the last quarter-century. Has any man ever perceived a 'real physical situation'?
How is it possible that a reasonable person could today still believe that he can refute our essential knowledge and
understanding by drawing up such a bloodless ghost?"
From Albert Einstein: Philosopher-Scientist,
Einstein's Reply to Criticisms
Published by Cambridge University Press, 1949.