Franpa said:
Ah, I assume they're substitutes? Thank you
No, they're the actual ingredients (or at least some of them).
Potions use a recipe that specifies the base ingredients needed to make it, such as rebis, quebith, caelum. These ingredients are NOT available in their pure form anywhere in the game, so you need to use an item that contains it.
This may be a plant extract (this is why you collect plants), something from an animal, a mutagen, or a mineral. When you make a potion, the top right section of the potions screen shows what you have in inventory with each base ingredient. The main inventory list also shows the icon for the base ingredient in the description of each item.
The system will make its own decision on which item to use. This can be dangerous, as it may pick something like an elemental stone that you need for crafting, so you should always check what it's going to use and substitute when necessary, before making the potion.