What matters is the intent, mens rea, the will to steal, and the intent to permanently deprave the rightful owner of his use of the item. This is critical. Without, you could become a thief without even wanting to steal anything, by mistake and accident, and I hope we can agree that this is not the intent of the law!
Allow me to show a counter example.
You are in a meeting and you have the habit of putting your cell phone on the table because it's uncomfortable in your pocket. After the meeting, you pick up your cell phone and go back, only to notice in your office that you forgot to take your cell phone along, it's on your office table, and you swiped the cell phone of someone else who just happened to have the same habit and the same phone model. Are you a thief?
According to your original statement a few postings up from this one,
"A thief takes something away from the owner dishonestly without the owner's consent, regardless of whether or not they plan on returning it."
you would be.
Maybe that's why they don't let you word laws.