Interesting. I thought your original comment was explaining why the IRS doesn't have all the information it needs to determine tax due by itself, and your examp

Interesting. I thought your original comment was explaining why the IRS doesn't have all the information it needs to determine tax due by itself, and your example was stock trading. In that case not only is there a documentary requirement at time of transaction, but there's a catch-all rule for cases where you turn out not to have such documentation. Amnesia or "ad hoc unrecorded environments" absolutely aren't exceptions to any of those rules. Perhaps you're trying to make a different point now, or I misunderstood your original one.