Much like psychic paper, the Doctor's psychic credit cards tricked machinery or onlookers into believing them something else. The Ninth Doctor gave one to Captain Jack to help him look after Das, the Neanderthal sent forward in time to Bromley in 2005. The technology to make psychic credit cards may have originated from the 51st century, as Jack was familiar with them, commenting that they were banned after the infinite recession on Baydafarn. (PROSE: Only Human)