The term philosophy comes from the Greek word “Φιλοσοφία” (philo-sophia), which means “love of wisdom”.The Penguin Dictionary of Philosophy defines it as the study of “the most fundamental and general concepts and principles involved in thought, action, and reality“.
 
Thales believed that there was an underlying unified system that could explain all things. He believed that all things are comprised of an active or ‘alive’ core, that water was the medium common to everything, that things change to and from and into each other, in a rational system of repetitive cycles. 
 
Thales relied on this idea of unified rational repetitive cycles to make the first exact prediction of the date of an eclipse. He also predicted the size of an olive crop harvest, benefiting personally by standing by with a large supply of wine presses to sell. 
 
More importantly, though, Thales was the first to do both of these using observation and deduction only, without referencing an outside god, goddess, religious or scientific system.

Immanuel Kant wrote his Critique of Pure Reason (1781/1787) in an attempt to reconcile the conflicting approaches of rationalism and empiricism and establish a new groundwork for studying metaphysics.

What were once philosophical pursuits have evolved into the modern day fields of psychology, sociology, linguistics, and economics (among others).
 
Just like psychology (science of psyche) which has evolved into a field of its own from philosophy, metaphysics can also be considered a realm of study in its own class because it is the science of spirit.
 
Everything subject can be fundamentaly considered as philosophy. We start primarily by thinking about something and contemplating on how it works and why it is so. This is how all science begins. We first have a concept or idea about something and then only later when the technology is developed can we deal with it in a more physically scientific way.
 
Even from the very early ages in history, the mind was talked about by Plato through his concept of the world of thoughts. Socrates emphasized the importance of the mind through his passionate discussions regarding reasoning and introspection.
 
Philosophy can be considered the very first subject of the mind.