 To make progress, Hume maintains, we need to “reject every system … however subtle or ingenious, which is not founded on fact and observation”. These systems, covering a wide range of entrenched and influential metaphysical and theological views, purport to have discovered principles that give us a deeper and more certain knowledge of ultimate reality. But Hume argues that in attempting to go beyond anything we can possibly experience, these metaphysical theories try to “penetrate into subjects utterly inaccessible to the understanding” (EHU 1.11/11), which makes their claims to have found the “ultimate principles” of human nature not just false, but unintelligible. These “airy sciences”, as Hume calls them, have only the “air” of science (EHU 1.12/12).