They are simply mental constructs of philosophy that have no objective existence in and of themselves.
The idealism of Kant was a direct reaction against the empiricism of philosophers such as Locke or Hume whose skepticism (if unchecked) could unmake the validity of all scientific inquiry in the minds of people (ibid.).
To understand the other side of the coin with Locke and empiricism, we must understand that his investigation into knowledge began with his asking how humans acquire the basic substances out of which that knowledge is composed that that makes up our ideas. To explain this, he formulated the concept of empiricism. According to Locke, empiricism is a theory of human knowledge that claims that knowledge comes about primarily via the phenomenon of sensory experience. Empiricism emphasizes the role of human experience and evidence. This is especially done via sensory perception, in the way of the formation of ideas, over their innate manifestations or traditions....
This approach emphasizes objective evidence, especially as gathered by scientific experiments using the scientific method. Experiments are a fundamental component of the scientific method that states that all hypotheses and theories have to be tested against the observations of the natural world (Kemerling, 2011).
Conclusion
This difference (and balance) in between the a priori and a posteriori capabilities of the mind is fundamental to the understanding of conscious thinking. The idealism and empiricism of Kant and Locke while emphasizing different aspects of knowledge have helped to get us to where we are now. However, empiricism has become more of the basis of the scientific method as we now know it.
References
A priori/a posteriori. (2012, January 21). Retrieved from http://humancond.org / analysis/philosophy/a_priori.
Kemerling, G. (2011). Locke: The origin of ideas. Retrieved from http://www.philosophypages.com/hy/4l.htm.
