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Philosophy into Religion 
While generally considered distinct, the lines between philosophy and religion are nebulous at best. Many historical examples exist of individuals who wrote philosophy (and from a philosophical perspective), but drew conclusions that could only be described as relating to religion. Nietzsche and Descartes provide two such examples:
- Nietzsche - Nietzsche, writing philosophically, declared that "Gott ist tot" (God is dead) based on the grounds that God has made it impossible for us to know Him. While this explanation of Nietzsche's arguments is overly simplistic, Nietzsche clearly used philosophical grounds to make an assertion into religion—and for Nietzsche, the natural result of his conclusion was the rejection of religion for nihilism.
- Descartes, writing several centuries before Nietzsche, was a philosopher and mathematician who developed several advanced theories which are still used today. One of Descartes' most significant contributions to Science has been the beginnings of the modern version of the Scientific Method (Descartes approached science and philosophy as a skeptic). Descartes referred to this as "la méthode" (the Method). In a philosophical work which he wrote called Méditations, Descartes explores the possibility of using the Method philosophically to explore whether or not God exists. Ultimately, Descartes reaches the conclusion that, rationally, God must exist on a philosophical/scientific basis. Descartes' application of Science to make a statement about Religion clearly crosses relosophical borders.
Religion as Philosophy 
Religions often fail to distinguish between religion and philosophy themselves, or contain clearly reflective and philosophical elements. Buddhism is an excellent example of this:
- Buddhism: Three of The Four Noble Truths of Buddhism, while considered central tenets of the religious practice of Buddhism, are essentially philosophical, and the fourth arguably so. An individual could reach the same understandings of the Four Noble Truths without ever touching on Buddhist theology or feeling any "religious" experience. However, these tenets are critical to the foundation of Buddhism.
- Many other religions contain truths which are philosophical in nature, though they may have a less central position within the theology of those religions.
Religion into Philosophy 
Not only can philosophy make forays into religion, but religions often shape or lead to new philosophical thought and discourse, often within the context of a religious background. Centuries of philosophy accompany Christianity and Islam, for example, and the relosophical background of most philosophers tints their perceptions and philosophies, even if that background is Agnostic or Atheistic in nature.
- Islamic Philosophy: After the introduction and spread of Islam in the Middle East, a vast "empire" of culture and learning arose, giving rise to a number of new philosophies and philosophers working within the religious context provided by the Qur'an. One significant example of this is Al-Ghazali's assertion that when fire and cotton are placed in contact, God, and not the fire, burns the cotton directly—Al-Ghazali, working from a religious viewpoint, then used logic to develop new philosophies within that context. Historically, such philosophies sometimes have become entirely separated from the religious context which may have spawned them, and become independent concepts in their own right. Even when they are not, it is clear that religion spawns thoughts that are clearly philosophical, and which are approached in a logical and rational manner.
- Christian philosophy: Christian philosophy is similar in some regards, though it arose in different contexts. Examples of the crossing of relosophical borders include the writings of Origen, who reconciled Platonism with Christianity, Martin Luther who refused to believe that a Church which violated its own doctrines could rationally contain God's truth, and Søren Kierkegaard, who wrote on a variety of philosophical topics which frequently touched the realms of theology.
- A less "orthodox" example is Maoist Philosophy—while not religious on its face, Maoism, by encouraging the establishment of a personality cult, shares many significant elements with other religions, and became the driving force for the development of Communist philosophy in China. In some other historical contexts, similar personality cults have actually marked the commencement of new religious movements.
The influence of religion on philosophy has been long and convoluted, but, in practice, one finds that it is very difficult to separate the two entirely, and some of the most influential philosophers in history (such as Descartes) have merged the two seamlessly, at least in their own minds.