Relative Bodhichitta is practised on the basis of the ordinary, conceptual mind and is perfectly possible to accomplish, even for a beginner, prodvided he looks within himself and practises properly. When this relative Bodhichitta has been perfected, moreover, absolute Bodhichitta, the wisdom of Vipashyana, the realization of no-self, arises by itself.
We can think of relative Bodhichitta as having two aspects: that of emptiness and that of compassion. To possess both compassion and an understanding of emptiness is like having wheels on one's car. If all four are present, the car is roadworthy; but if a wheel is missing, it is impossible to go anywhere. Meditation on emptiness without compassion is not the Mahayana path; meditation on compassion where the aspect of emptiness is lacking is not the path either. We need both emptiness and compassion together.
From Enlightened Courage: An Explanation of the Seven-Point Mind Training by Dilgo Khyentse Rinpoche, translated by Padmakara Translation Group, pages 10-11.