The answer is his fervent. With intense requires another time of quiet understanding, President Reagan and Mikhail S. Gorbachev today marked the main bargain lessening the measure of their countries' atomic arms stockpiles. The President and the Soviet pioneer, starting three days of talks went for considerably more extensive diminishments, swore to expand on the agreement by endeavoring toward what Mr. Gorbachev called ''the more vital objective,'' decreasing long-go atomic weapons.