Nothing is impossible, the word itself says 'I'm possible'

We checked Hepburn's autobiographical material, published interviews, speech records, and the standard biographies, and found no instance of her ever saying or writing this line — no primary source connects it to her at all. What the record does show, via Quote Investigator's tracing of the phrase, is that the "impossible / I'm possible" wordplay circulated in motivational and self-help contexts and only became attached to Hepburn's name well after her death in 1993. The precursors of the pun are largely anonymous or belong to earlier inspirational writing, and while various names have been floated over the years, no single competing author can be cleanly and credibly identified as the definitive originator of this exact phrasing. So we have a solid positive reason that this is not Hepburn's — the recognizable modern form is documented later and in other contexts — but we lack one named alternative author to pin it on. Weighing the strong evidence that the line postdates Hepburn and arose in unrelated motivational literature against the absence of a single clean rival claimant, the honest reading is that the wordplay predates and exists independently of its Hepburn attribution, which is documented positive grounds beyond mere absence of a source.