diff --git a/MessagingApp.markdown b/MessagingApp.markdown
index f620808..9b1cea7 100644
--- a/MessagingApp.markdown
+++ b/MessagingApp.markdown
@@ -13,16 +13,16 @@ MessagingApp uses a single channel with a random identifier:
 
 ### Validation
 
-* A message of type PRIVATE_MESSAGE is valid if it is well-formed.
-* A message of type ATTACHMENT is always valid.
+* A PRIVATE_MESSAGE is valid if it is well-formed.
+* An ATTACHMENT is valid if it is referenced by a valid PRIVATE_MESSAGE.
 
 ### Storage policy
 
 * All valid local messages are stored.
 * All valid remote messages are stored.
-* Messages of type ATTACHMENT that are not referenced by messages of type PRIVATE_MESSAGE are garbage-collected.
+* An ATTACHMENT that is not referenced by a valid PRIVATE_MESSAGE may be garbage-collected.
 
 ### Sharing policy
 
-* Each local message is shared with one remote device.
+* Each valid local message is shared with one remote device.
 * Remote messages are not shared with other remote devices.