The Heart Is Its Own Reason marks the dazzling debut of a new voice in Canadian fiction. With a surgeon's skill, Natalee Caple reveals the workings of the human heart and the eternal search for love. In the title story, Lana, nursing her infant son, tells her friend Karen: ''Men are a kind of blade. Even though you can see how sharp and quick they are against another woman's skin it still surprises you when you embrace them and they cut, cut through you, escaping out of your opened back.'' But it is Lana who wielded the knife, stabbing her lover Gary, and playing Miles Davis as he slowly bled to death. The police can understand a crime of passion, but not the music and not the darker terror that drove her to murder. These 15 stories cut to the bone, probing the impact of passion between mothers and children, sisters and brothers, husbands and wives. The results are poignant and often touched by loss. In ''The Price of Acorn,'' a boarder offers a couple a washing machine in return for their only child; in ''Under Hunger,'' a young girl learns to read in her instructor's bed; in ''Breathing in April,'' a novelist plays out her own Death in Venice fantasy as she follows a stranger to the beach.Tautly written, each of these stories challenges the traditional forms of love, while never denying its powerful nature - its joy and its often desperate conclusions.