Youâ€™ll still pay all costs for that spell, including additional costs. You may also pay alternative costs such as that granted by Jodah, Archmage Eternal.
You must follow the normal timing permissions and restrictions of the cards you play from your library.
The top card of your library isnâ€™t in your hand, so you canâ€™t cycle it, discard it, or activate any of its activated abilities.
The card youâ€™ll exile from the top of your library is determined as Precognition Fieldâ€™s last ability resolves. This might not be the card that was on top of your library when you activated that ability.
Precognition Field lets you look at the top card of your library whenever you want (with one restrictionâ€”see below), even if you donâ€™t have priority. This action doesnâ€™t use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.
If the top card of your library changes while youâ€™re casting a spell, playing a land, or activating an ability, you canâ€™t look at the new top card until you finish doing so. This means that if you cast the top card of your library, you canâ€™t look at the next one until youâ€™re done paying for that spell.