diff --git a/docs/project/principles/information_accumulation.md b/docs/project/principles/information_accumulation.md
index 5cf324ba70c1..c43023e26f6c 100644
--- a/docs/project/principles/information_accumulation.md
+++ b/docs/project/principles/information_accumulation.md
@@ -79,6 +79,10 @@ especially important to the coherence of generics and templates.
 -   When an `impl` needs to be resolved, only those `impl` declarations that
     appear earlier are considered. However, if a later `impl` declaration would
     change the result of any earlier `impl` lookup, the program is invalid.
+-   Declarations in an API file that are insufficient for use by clients that
+    import the library require a definition in the API file.
+    -   Where possible, we should make declarations sufficient, so a definition
+        is not required.
 
 ## Exceptions