That cannot undergo precise evaluation: imponderable problems. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "imponderable" in a sentence
"We have no knowledge that the luminiferous ether is attracted by gravity; it is sometimes called imponderable because some people vainly imagine that it has no weight; I call it matter with the same kind of rigidity that this elastic jelly has."
"In effect, then, the physicist has dispossessed the many imponderables in favor of a single imponderable -- though the word imponderable has been banished from his vocabulary."
"There are far too many factors-too many that are "imponderable" - whose weight just cannot be estimated."