Imperceptible, insensible, impalpable, intangible, inappreciable, imponderable all mean incapable of being apprehended by the senses or intellect (as in form, nature, extent, or degree) even though known to be real or existent.
(Except for this denial of apprehensibility, these terms carry the same implications and connotations as the affirmative adjectives discriminated at PERCEPTIBLE ).