Back in the day when I was playing RM, with 3 realms, I had 3 separate rationales for "magic item intelligence".
1. Mentalism: You can invest an item with the mental pattern of any Mind Stored target. It won't make the item necessarily as intelligent as the target copied, and certainly won't translate the same level and skill sets, but the item will have an identical mindset, and will react in ways eminently predictable to those who know the original person well.
2. Channeling: You can invest an item with the spirit of any willing individual. Whether this individual is alive, dead, whatever, varies according to the ethics of the religion in question. The process is similar to the creation of undead, but avoids most of the moral complications. The item's level, intelligence and skill set depends on both the abilities and intelligence of the spirit chosen and how complete the investiture is. In short, you may or may not get all the skills and abilities of the original spirit (varies with the skill of the craftsman), but you certainly won't get an item more intelligent or more skilled than the spirit invested in it.
3. Essence: Unlike the other methods, intelligent items made using Essence magic don't use an already existing intelligence as a template. What laymen call "intelligence" and "personality" in an Essence item, the creator himself terms "heuristics". It's a complicated and delicate process, but creates an "intelligence" (defined as a pattern that constantly reprograms itself in response to environmental sampling) that is unique and non-spiritual, thus avoiding many of the issues of the other 2 methods.