Joseph Agassi's defense of speculation and of metaphysics is the result his overall conception of philosophy and his epistemological stance. Philosophy for Agassi is the "merciless' search for principles which invariably ends in the central tenant of critical rationalism: "we are all in error." Critical rationalism is the relentless search for truth through the testing of ideas. Philosophy thus conceptualized can abide neither Karl Popper's fideism nor Wittgenstein's mysticism as both delimit rationality, resulting in a kind of pseudo-rationalism. Because rationality must be relentless, it seeks ideas from all sources and tests them, including the metaphysical.