  We present three protocols for verifying all languages: (i) For any unary
(binary) language, there is a log-space (linear-space) interactive proof system
(IPS); (ii) for any language, there is a constant-space weak-IPS (the
non-members may not be rejected with high probability); and, (iii) for any
language, there is a constant-space IPS with two provers where the verifier
reads the input once. Additionally, we show that uncountably many binary
(unary) languages can be verified in constant space and in linear (quadratic)
expected time.
