Surah 69. Al-Haqqah, Ayah 51

All Truth is in itself. But as received by men, and understood with reference to men's psychology, certainty may have certain degrees. There is the probability or certainty resulting from the application of man's power of judgment and his appraisement of evidence. This is ilmulyaqin, certainty by reasoning or inference. Then there is the certainty of seeing something with our own eyes. "Seeing is believing." This is 'ain-ul-yaqin, certainty by personal inspection. See cii. 5, 7. Then, as here, there is the absolute Truth, with no possiblity of error of judgment or error of the eye, (which stands for any instrument of sense-perception and any ancillary aids, such as microscopes etc.). This absolute Truth is the haqq-il-yaqin spoken of here.