Hubbard further defined perversion in his 1951 book "Science of Survival: Prediction of Human Behavior", where he introduced the concept of the "tone scale", a means of classifying individuals and human behavior on a chart running from +40 (the most beneficial) to -40 (the least beneficial). Sexual perversion, a category in which he included homosexuality, was termed "covert hostility" and given a score of 1.1, "the level of the pervert, the hypocrite, the turncoat, ...the subversive." He considered such people to be "skulking coward who yet contain enough perfidious energy to strike back, but not enough courage ever to give warning."