The notion of identical gives rise to many philosophical problems, including:
1. What does it mean for an object to be the same as itself?
2. If x and y are identical (are the same thing), must they always be identical? Are they necessarily identical?
3. What does it mean for an object to be the same, if it changes over time? (Is applet the same as applet+1?)
4. If an object's parts are entirely replaced over time, in what way is it the same?