As Evan Thompson (2007) explains, external (‘transcendent’) events are ‘given as such by virtue of the intentional activities of consciousness.’ In this sense they really are external, yet they are ‘intentionally immanent’:
their status as external events for the system (as opposed to their status for an observer of the system) is a function of the system’s own activity. Their meaning or significance corresponds to an attractor of the system’s dynamics (a recurrent pattern of activity toward which the system tends), which itself is an emergent product of that very dynamics. The external world is constituted as such for the system by virtue of the system’s self-organizing activity.
— Thompson (2007, 27)
This is another way of saying that an external event (of which a system is conscious) is the dynamic object which determines a sign to determine an interpretant which is an event internal to the system’s dynamics. This is how the Thirdness of a sign ‘brings about a Secondness between two things,’ as Peirce put it; it makes one manifest as external to the other.