Mike: Tom did not tell me that I could use his computer, but it would not be wrong for me to use it anyway.
Conclusion
Even though Tom didn't tell me I could use his computer, it would be not wrong for me to do so.
Evidence
Last week Tom used Mary's bike even though she never told him he could use it.
Evaluate
Why are we allowed to take Tom's computer without asking? Because we've seen Tom take somebody else's stuff without asking before.
If we've ever seen someone toilet-paper someone else's house, then it's not wrong to toilet paper their house.
What is the abstract principle we're using here?
Goal
Look for something like,