Allah's Signs on the Earth and in Mankind
وَفِي الْأَرْضِ آيَاتٌ لِّلْمُوقِنِينَ (And in the earth, there are signs for those who [ seek truth to ] believe,... 51:20). In the preceding verses in the first instance a description of the infidels and rejecters and their evil fate was given. As against those who flounder in the welter of wrong beliefs and evil deeds and deny the possibility of the Day of Judgment, the qualities of the believers and the righteous were subsequently given, and their elevated position was described. Now once again attention is drawn to those who deny the possibility of Judgment Day. They are invited to ponder on the signs of Allah on earth that testify to the might of the Creator and His boundless power, and thus the skeptics are advised to desist from denying the Judgment Day. In this way the statement is connected to the preceding statement in verse [ 8] "You are involved in contradictory statement."
Tafsir Mazhari is of the view that this verse too describes the quality of the believers and the righteous. The word muqinin (those who have certainty of faith) refers to the same God-fearing people mentioned in an earlier verse. The sense is that these people keep pondering on the divine signs spread on earth and in their own beings, and it increases their belief. In another verse they are described as وَيَتَفَكَّرُونَ فِي خَلْقِ السَّمَاوَاتِ وَالْأَرْضِ "...and ponder on the creation of the heavens and the earth...". (3:191)
Allah's signs on earth include what Allah has placed on the earth, the various plants, animals, valleys, mountains, deserts, rivers and oceans. He also created mankind with different languages, colours, intentions and abilities, and a variety among them, differences in the power of understanding and comprehension, their deeds, and ultimately earning happiness or misery. Allah put every organ in their bodies in its rightful place where they most need it to be.