impalpable
     adj 1: incapable of being perceived by the senses especially the
            sense of touch; "the intangible constituent of
            energy"- James Jeans [syn: {intangible}] [ant: {tangible}]
     2: imperceptible to the senses or the mind; "an impalpable
        cloud"; "impalpable shadows"; "impalpable distinctions";
        "as impalpable as a dream" [ant: {palpable}]
     3: not perceptible to the touch; "an impalpable pulse"
Source: WordNet® 2.0