Imam Al-Ghazali says in his monumental work, “ The Alchemy of Happiness.”
“As regards the joys of heaven and the pains of hell which will follow this life, all believers in the Quran and the Traditions are sufficiently informed. But it often escapes them that there is also a spiritual heaven and hell. Concerning the former, of which God said to His Prophet, “ Eye hath not seen, nor ear heard, neither hath it entered into the heart of man to conceive the things which are prepared for the righteous.”
In the heart of the enlightened man, there is a window, opening on the realities of the spiritual world, so that he knows not by hearsay or traditional belief, but by actual experience, what produces wretchedness or happiness in the soul, just as clearly and decidedly as the physician knows what produces sickness or health in the body. He recognizes that knowledge of God and worship are medicinal and that ignorance and sin are deadly poisons for the soul. Many even so-called “learned” men, from blindly following others’ opinions have no real certainty in their beliefs regarding the happiness or misery of souls in the next world, but he who will attend to the matter with a mind and heart unbiased by prejudice will arrive at clear convictions on this matter.”