Yes you can; if you have permissions.
I drew a reference guide to chmod once and I always refer to it:
OWNER GROUP WORLD
r w x r w x r w x
1 1 1 1 0 1 1 0 1
7 5 5
As long as you can count in binary it makes perfect sense.
This way you can set precise: r
rite or ex
ecute access levels
to files based on: owner, group or world
where or when ever you want.
So read only would be 100 or 4.