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1.
1. The maturity of your emotions.

2. The extent of which you understand your partner.
1. eg. His heart knowledge was lacking to such extent that he rocked up to his anniversary with another woman.

2. eg. His heart knowledge was so great for her, that he knew how to comfort her in the event of her father's death.

eg. her heart knowledge was so limited that she didn't notice his absence of smile.