Unknowable vs unknown

This is from Crafting Retirement Income That Is Stable, Secure, and Sustainable by Jason K. Branning, CFP®; and M. Ray Grubbs, Ph.D. [2017 SSRN]

Planning against the unknowable offers a different starting point than simply planning for unknowns. We can know that a client will ultimately die, but it is not just unknown as to when, but individually unknowable. The distinction between unknown conditions and unknowable conditions is an important one for financial planners and clients. Unknown implies that there is something that can be done to provide knowledge on which actions can be taken. Unknowable implies that there is nothing that can be done to add knowledge to a particular client situation. The unknowable aptly describes individual client issues such as longevity and conditions within longevity. Conducting empirical tests requires assumptions or constraints to get a meaningful mathematical solution. Math is useful if conditions can be controlled and limited, as math measures quantity.