Believe you can and you're halfway there

This crisp bit of encouragement is one of the most reproduced lines credited to Theodore Roosevelt, yet no primary source for it has ever been located. The Theodore Roosevelt Center, which maintains the digital archive of his papers, states that while the line is frequently attributed to him, it has not been found in his correspondence, speeches, or other writings; the Theodore Roosevelt Presidential Library likewise reports that no primary source has ever surfaced. Our own targeted search of his minor and posthumously collected works and his published letters turned up no verbatim appearance, and vague online references to a Roosevelt "sayings" collection around 1907 come with no verifiable citation. The thought does square with Roosevelt's lifelong gospel of self-confidence and the strenuous life, and no rival author has been identified — so this is not a case of misattribution, simply one of an unsourced maxim. We therefore treat it as attributed: broadly and consistently credited to Roosevelt, but without a documented original in his own hand.