The first line of the third quatrain extends the conceit of the Platonic theory, the idea that the perceptions of reality are merely reflections of the essential reality of forms. Platonic theory suggests that our perceptions are derived from this world of forms in the same way shadows are derived from the objects that are lit. The metaphor of shadow was often employed to help explain the illusory quality of perception and the reality of forms, both by Renaissance Platonists and by Plato himself in his book, Symposium. In the sonnet, spring can only offers shades of the beauty of the youth. The youth is presented as the ideal Beauty, the form, from which all other beautiful things come. This idea is summarized in line thirteen of the sonnet: "In all external grace you have some part." This line finds the youth to be the exclusive source of all beautiful things, expanding his "domain" even further than the first quatrains in which the youth is said to be the source of the legendary figures of Adonis and Helen.