It is essential for the Kantian programme that it can develop principles for the seeking and finding of knowledge. This requires Kant to combine the openness that is required for discovering genuinely novel knowledge with the necessity provided by principles. This combination of extreme methodological openness with strong principles should add to our understanding of Kant’s position vis-à-vis empiricism and rationalism. It will be shown that Kant indeed develops an open methodology that is intended to give direction to our cognitive practices without determining their results. This implies a revision of the standard understanding of ideas of reason in their regulative use: Kant’s imagery of “horizons” and “mirrors” suggests that, in principle, all concepts can function as regulative ideas. In the absence of clear ways of categorizing philosophers as either ‘empiricists’ or ‘rationalists’ in Kant’s period, these methodological issues help consolidate our picture of how Kant positions himself within the field of options that became labelled by these terms.