Full Idea
Negative consequences make the extension of the consequences of our actions indefinite, and this means that it is difficult to assess them; it may make it impossible, since there is now no clear sense to the idea of THE consequences of an action at all.
Clarification
'Negative consequences' are what does not happen, because of the action
Gist of Idea
Negative consequences are very hard (and possibly impossible) to assess
Source
Gordon Graham (Eight Theories of Ethics [2004], Ch.7)
Book Reference
Graham,Gordon: 'Eight Theories of Ethics' [Routledge 2004], p.139
A Reaction
The general slogan of 'Do your best' covers most objections to the calculation of consequences. It is no excuse for stealing a wallet that 'at least I wasn't committing genocide'. How easy were the alternative actions to do?