Young people believe efforts to reduce pollution, poverty, and war are doomed to failure.
Conclusion
We must enable our children to believe that better futures are possible.
Evidence
Young people are pessimistic about reducing pollution, poverty, and war. This pessimism is harmful because people lose motivation to work for goals they think are unrealizable. We must prevent this loss of motivation.
Evaluate
The argument moves from "we must prevent the loss of motivation" to That step only works if enabling that belief actually does prevent the loss of motivation. Negation: if enabling the belief does not prevent the motivation loss, the recommendation doesn't serve the goal — and the conclusion fails.
Goal
Find the assumption linking "enable belief in better futures" to "preventing loss of motivation." Most directly: enabling that belief actually does prevent the loss of motivation that pessimism causes.