Say you use an encryption method to encipher the first page of A Tale of Two Cities. Someone else is tasked with determining what the plaintext was.
If their methodology is to (1) choose some method by which the ciphertext is transformed into a candidate plaintext, and then (2) apply that method, they will not be able to produce a candidate plaintext which (a) makes sense but (b) is not the actual original plaintext.
The exception is one-time-pad encryption, which isn't really worth noting. It's an exception because the decryption method for one-time-pad encryption is isomorphic to "choose what you want the plaintext to say, and then write down the key that will produce that plaintext". If the key is shorter than the ciphertext, this can't happen.
But "a property of all encryption methods other than OTP" is a less convenient way to say "a property of all encryption methods".
Say you use an encryption method to encipher the first page of A Tale of Two Cities. Someone else is tasked with determining what the plaintext was.
If their methodology is to (1) choose some method by which the ciphertext is transformed into a candidate plaintext, and then (2) apply that method, they will not be able to produce a candidate plaintext which (a) makes sense but (b) is not the actual original plaintext.
The exception is one-time-pad encryption, which isn't really worth noting. It's an exception because the decryption method for one-time-pad encryption is isomorphic to "choose what you want the plaintext to say, and then write down the key that will produce that plaintext". If the key is shorter than the ciphertext, this can't happen.
But "a property of all encryption methods other than OTP" is a less convenient way to say "a property of all encryption methods".