Sayyidi wa sanadi Mufti Mohammed Taqi Usmani (Allah protect & preserve him. Ameen!) wrote,

(12) The verse signifies that one must not follow whims and conjectures, nor should he act on presumptions based on surmises. He should act on premises based on knowledge. If someone believes in conjectures without proper investigation, he will be questioned on the Day of Reckoning on what basis he accommodated this belief in his heart, and whether he believed in something after seeing it with his own eyes, or hearing it with his ears. If he believed in it without proper investigation through these sources of knowledge, he will be punished.

The Meanings of Noble Quran, Chapter 17-Isra