What principle or idea is the speaker trying to convince his son to accept in the poem "If" by Rudyard Kipling?
The speaker is trying to convince his son that being a man means leading a life of integrity and having depth of character. Many of the admonishments the speaker presents are based on a Judeo-Christian moral code. For example, being patient, not dealing in lies, not hating those who hate you, not being vain or arrogant ("don't look too good, nor talk too wise), not complaining ("never breathe a word about your loss"), being industrious ("fill the unforgiving minute with sixty seconds’ worth of distance run"), and keeping your virtue are all qualities that religious people would agree are important for a man to have.
Other pieces of advice the father gives have to do with being strong in the face of hardship and overcoming adversity with grace. Keeping your head, trusting yourself, using dreams to motivate but not distract you, not becoming discouraged when things don't go your way or when people betray you, being able to take risks, and having good relationships with people are all behaviors a person develops by having a wide variety of experiences—including both successes and failures—and responding properly to them. The father's lesson that he wants his son to learn is that manhood is not achieved at a certain age; rather, it is attained after someone has developed a track record of acting with integrity and building strong character in the face of the vicissitudes of life.
The main principle emphasized in Kipling's "If" is the importance and value of one's perceptions, reactions and the overall value of self sufficiency. The author is trying to show that a lot of the things in life are heavily influenced by how you react to them. This poem also makes a strong case for the importance of balance in life. If you can handle the bad things in life with the same grace that you handle the good, you'll go far and do well.