In case of a person, the external sensory data provides the grounding. Consider a prisoner who spent a long time in hole in the cell, he starts hallucinating due to no sensory information to ground his neuronal firings.
Philosophically speaking, sensory data is no more "external" or grounded than words are. You do not see - your eyes see. You do not hear - your ears hear. You cannot truly interact with the world except at a remove, through various organs. You never perceive the world "as it really is". Your brain attempts to build a consistent model that explains all your sensory input - including words. Where words and sensory input disagree, words can even win - I can tell you that you are in VR, or dreaming, or that immigrants are the cause of all your problems, and if I am careful and charismatic you may believe me...
tl;dr what you think of as "grounding" is just yet more relative context...
