Two different classes of identifiers must be tested to reliably authenticate things and people: assigned identifiers, such as names, addresses and social security numbers, and some number of physical characteristics. For example, driver’s licenses list assigned identifiers (name, address and driver’s license number) and physical characteristics (picture, age, height, eye and hair color and digitized fingerprints). Authentication requires examining both the license and the person to verify the match. Identical things are distinguished by unique assigned identities such as a serial number. For especially hazardous or valuable things, we supplement authentication with checking provenance — proof of origin and proof tampering hasn’t occurred.