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...