I have had no experience of what you have mentioned [regarding prophethood]: how, then, can I know that it exists and is verifiable, even though I admit its possibility?
You do not limit yourself to believing what you have experienced. On the contrary, you have listened to the reports of experienced men and have unquestioningly accepted their statements. Listen, therefore, to the utterances of the prophets: for they have indeed experienced and seen what is true in all that revelation has brought us. Follow in their path, and you will perceive some of that by direct vision.
Even if you have had no such experience, your reason peremptorily judges it necessary to believe and follow the experienced. Let us suppose the case of a man of mature mind who has never experienced sickness, and then he falls sick. He has a sympathetic father skilled in medicine, whose claim to be versed in medicine the sick man has been hearing ever since he reached the age of reason. His father compounds a remedy for him and says: ‘This is good for your sickness and it will heal you of your malady.’ What, then, does the sick man’s reason require, if the remedy be bitter and foul-tasting? That he should take it? Or that he should disbelieve and say: ‘I do not understand this medicine’s suitability for obtaining a cure, since I have had no experience of it’? Undoubtedly you would regard him as stupid if he acted thus.
So, too men of insight regard you as stupid in your hesitation to believe. If you then say: ‘How can I know the compassion of the Prophet–God’s blessing and peace be upon him!–and his knowledge of this spiritual medicine?’ I would say: ‘And how did you know the compassion of your father, seeing that it is not something perceptible to the senses? Rather, through the indications of his various attitudes and the evidences of his actions in his daily comings and goings you came to know it with a necessary and unquestionable knowledge.’
Anyone who reflects on the sayings of the Apostle–God’s blessing and peace be upon him!–and on the reports that have come down about his concern for guiding men rightly and his subtlety and delicacy in drawing people by the various forms of gentleness and kindness to the improvement of their morals and the patching up of discord and, in general, to whatever is best for their religious and temporal affairs, obtains a necessary knowledge of the fact that the compassion of the Apostle for his Community was greater than a father’s compassion for his son.
Moreover, when one considers the marvelous deeds manifested at his hands, and the wonders of the unseen reported in the Qur’an and the traditions, and what he mentioned about the distant future–which in the event turned out just as he had said–he knows with necessary knowledge that the Apostle had reached the stage which is beyond reason and that the eye had been opened for him to which are unveiled the unseen and the special properties and things which reason does not perceive…So try it yourself and meditate on the Qur’an and study the traditions–then you will know that by seeing with your own eyes.
Source: Deliverance from Error (al-Munqidh min al-Dalal) by Al-Ghazali (translated by R.J. Mccarthy, S.J.), pp. 77-79. Please note that the title given is of my own and not included in the actual work.