* Magical Abilities: Even as a baby, Harry showed magical abilities (like accidentally making Dudley float). These abilities wouldn't have gone unnoticed.
* Magical Lineage: His parents were both powerful wizards, and the Ministry of Magic is quite strict about enforcing wizarding laws. They wouldn't have allowed a child with such a magical heritage to go unmonitored.
* Blood Status: Though the books emphasize the importance of blood purity, this was mainly an ideology of the Death Eaters. The Ministry of Magic doesn't necessarily care about blood status when it comes to magical education.
So, if Harry hadn't received an invitation, it's likely he would have been:
* Visited by a Ministry of Magic official to assess his abilities and enroll him at Hogwarts.
* Placed in a wizarding orphanage until he was old enough to attend school.
However, the books don't explicitly state what would have happened, so it's also possible that Harry:
* Could have developed his magic secretly and perhaps even been discovered by another magical creature or a wandering wizard.
* Might have been sent to a muggle school, living with the Dursleys as a normal child, but perhaps developing his magic secretly.
It's a fascinating thought experiment, and it highlights how even in the magical world, fate can sometimes play a powerful role.