The same quality repel each other, the opposite attract each other. Then, simply difference don’t repel and attract. Another say, simply difference repel by half, and attract by half. Namely, simple difference fluctuate between repulsion and attraction. And the same quality gathers each other, the same quality lose each other. And the opposite gathers each other, it produce new quality. For example, two persons want a piece of cake, two persons oppose each other. And the person who have one apple, but he want orange. Another person who have orange, but he want apple. So, two person attract each other and they exchange apple and orange each other. There is harmony. So saying conversely, two repel each other, they have same quality, and two attract each other, they have opposite quality.