Keats' concluding line to "Ode on a Grecian Urn" represents much of his thinking and the ideas behind Romanticism. As he is staring at this urn, the speaker (presumably Keats) is engaged in a quest to understand the ideas of truth, beauty, love, and identity. As he has stared at this urn, he understands that there is no set of higher principles or dogma to determine what truth is. For Keats, there is no concrete and singular set of principles that explains what constitutes beauty. Being the Romanticist he is, Keats is inclined that truth and beauty are interlinked, and signified by the urn at which he is staring. Rather than spend his time in the elusive and hopeless pursuit of a set of standards that define for individuals what constitutes beauty and truth, Keats determines that individuals can find beauty in truth and can find truth in beauty. As he has studied the Urn in the poem, this is what he has discovered: "Truth is beauty and beauty truth." This is all he knows and all he has to know in order to live a meaningful life.
The concluding lines of the poem highlight a major tenet of Romanticism. Specifically, individuals are the authors of their own destiny. Social conventions, religious dogma, and external standards that seek to reduce human choice and freedom are not the best determinants of truth and beauty as individual passion is. For Keats, this antiquated object contains more value on the level of truth and beauty than all the philosophical treatises, religious belief, and socially dictated notions of the good. As an artist of Romanticism, Keats believes that all people, especially artists, have an obligation to seek out this element of beauty and truth in their pursuits.
This quote also deals with the complicated argument of where truth comes from. The argument of John Locke, for instance, claims that all knowledge comes through the sense; there is nothing in our knowledge that does not come from sensual experience. The idealists/romantics believe that in addition to "tuition" we had a built in "intuition" that gave each of us direct access to knowledge. Thus, instead of arriving at truth through the additive process of acquiring sense knowledge, in the case of truth, we can intuit it directly through our appreciation of beauty. He states that if a thing is beautiful it is, by definition true; that which is true, must be beautiful. There is no scientific definition of what the "beautiful" is; but if we can get to know the truthful through the beautiful, then this knowledge is available to all of us, not just the educated.
This is an essential component of Romanticism; through our direct interaction with then natural world, natural beauty, we can attain a knowledge of the true. "That is all/ Ye know on earth, and all ye need to know."