In his theological writings Anselm sought to defend the faith by intellectual reasoning, his leading work being the ‘Proslogion’, in which he advanced his famous ‘ontological argument’. This proposed that if God is that than which nothing greater can be thought then God must exist, because if He were purely a mental concept it would always be possible to trump this concept by supposing it to exist in reality.
But he maintained that while it was the duty of Christians to exercise their intellects in the apprehension of revealed truth, the right use of reason was conditioned on faith. This is reflected in the following lines from the ‘Proslogion’: “I do not seek to understand so that I may believe, but to believe so that I may understand.” He died in 1109. His ‘day’ is 21st April.