Summa theologiae IIaIIae 66: On theft and robbery

Here there are nine things to consider:

1. Whether it is natural for man to possess external things

2. Whether it is lawful for anyone to possess something as his own

3. Whether the nature of theft lies in taking someone else's property secretly

4. Whether theft and robbery are sins of different species

5. Whether all theft is sinful

6. Whether theft is a mortal sin

7. Whether it is lawful to steal by reason of necessity

8. Whether all robbery is a mortal sin

9. Whether theft is a more grievous sin than robbery

articulus 1: Whether it is natural for man to possess external things

It seems that it is not natural for man to possess external things.

obiectio 1: For no man should claim for himself that which belongs to God. But dominion over all creatures belongs to God, according to Psalm 24:1: ‘The earth is the Lord's’, etc. Therefore it is not natural for man to possess external things.

obiectio 2: Moreover, Basil, expounding the words of the rich man at Luke 12:18, ‘I will gather all my fruits and my goods’, says: ‘Tell me: Which things are yours? Whence did you call them forth when you brought them to life?’ But whatever a man possesses naturally he can properly call his. Therefore man does not naturally possess external goods.

obiectio 3: Moreover, according to Ambrose in the book De Trinitate, ‘ownership is a title of power’. But man has no power over external things, for he cannot bring about any change in their nature. Therefore the possession of external things is not natural to man.