From an early age I instinctively thirsted after truth. This was part of my temperament, given by God, and was not a matter of choice or effort. So as I reached adulthood, I refused to accept ideas on the authority of others and the beliefs which I had been taught as a child lost their grip on me. I observed that the offspring of Christians grew up to be Christian, the offspring of Jews grew up to be Jews, and the offspring of Muslims grew up to be Muslims. I was driven to search for the inner truths which lie behind all religions.
It was self-evident to me that knowledge can only be certain if the object of knowledge is disclosed in such a way as to dispel all doubt. Thus I considered the various kinds of knowledge which I already possessed, and realized that none of it was certain, except the perceptions of the senses. I then wondered whether even the perceptions of the senses are open to doubt; and after profound thought I concluded that they are. For example, the shadow cast by the gnomon of a sundial may appear to the sense of sight to be stationary; yet by observing it hour by hour one discovers that it is moving.
I was now sceptical of all knowledge; and this total scepticism was a spiritual disease.
(Imam Al-Ghazali)