Where endless conjectures supply the defect of unattainable knowledge

From Lord Chesterfield, Letter XCIV in Chesterfield's Letters to His Son. Describing the researches of Lord Bolingbroke.

The common bounds of human knowledge are too narrow for his warm and aspiring imagination. He must go 'extra flammantia maenia Mundi', and explore the unknown and unknowable regions of metaphysics; which open an unbounded field for the excursion of an ardent imagination; where endless conjectures supply the defect of unattainable knowledge

Where the frontier of facts stops, it is worthwhile but dangerous to explore the wilderness of conjectures.