diff --git a/docs/contracts/functions.rst b/docs/contracts/functions.rst
index 7d1ce7256..ff49e4f28 100644
--- a/docs/contracts/functions.rst
+++ b/docs/contracts/functions.rst
@@ -222,6 +222,9 @@ Pure Functions
 --------------
 
 Functions can be declared ``pure`` in which case they promise not to read from or modify the state.
+In particular, it should be possible to evaluate a ``pure`` function at compile-time given
+only its inputs and ``msg.data``, but without any knowledge of the current blockchain state.
+This means that reading from ``immutable`` variables can be a non-pure operation.
 
 .. note::
   If the compiler's EVM target is Byzantium or newer (default) the opcode ``STATICCALL`` is used,