John Wesley used four different sources in coming to theological conclusions. These sources were first referred to as the Wesleyan Quadrilateral in 1964 by theologian/scholar Albert C. Outler in a collection of Wesley's works edited by Outler entitled simply John Wesley. The four sources are:
Scripture refers to the inspired and sufficient written word of God, the 66 canonical books of the Bible. Upon this foundation church traditions are adopted which agree with the biblical text, and those which are inconsistent are discarded. Next, reason is received as a gift intrinsic to the image of God. Finally, with Scripture, tradition and reason in place, human beings are prepared to experience the reality of God's love. 
It is interesting to note the reflection of Albert Outler on his formulation, "There is one phrase I wish I had never used: the Wesleyan Quadrilateral. It has created the wrong image in the minds of so many people and, I am sure, will lead to all kinds of controversy." 
The problems he anticipated come when the Quadrilateral is seen as "equilateral," and all four "sources" for authority and decision-making are seen as equally weighted. This was not Outler's intent nor Wesley's method. Rather, Scripture is to be viewed as the centerpiece from which the other sources are suspended.
However, misuse of the methodology is already manifest in many quarters on many issues:
It has been used to endorse same sex marriage in The Bible and Homosexuality and Same Sex Marriage where the latter says, "As I understand this Quadrilateral, Christians have four sources in any search for standards in making decisions: 1) The Bible; 2) Tradition, including but not limited to the tradition of the Church; 3) Reason, by which is meant science and the best of human rationality applied to the problem; and 4) Personal experience." He then states that "The Bible gives us almost no direct guidance in the issue of same-sex marriage," and proceeds to justify it on the basis of reason and experience.
It has been used to endorse the role of women elders and pastors in Changing Female Religious Leadership in Christianity where it is stated that, "The Methodist Quadrilateral uses reason or rationally critical perspective when it thinks about the Scripture and the traditions. . . . In many cases female leadership has been restricted by biblical and cultural restrictions. But the authority of experience has transformed the gender boundaries of religious leadership through American Methodist history."
These examples are not intended to judge the particular conclusions but merely to evidence the arrival at theological judgments which essentially eliminate Scripture as relevent.
- Albert C. Outler, John Wesley (ISBN 0195028104)
- The Wesleyan Quadrilateral in John Wesley by Albert C. Outler
- The Wesleyan Quadrilateral in the American Holiness Tradition by Leon Hynson
- John Wesley on the Authority & Interpretation of Scripture by Scott Jones
- Rethinking the Wesleyan Quadrilateral by Paul Wesley Chilcote
- What are the Wesleyan Distinctives that Shape & Inform Christian Higher Education Today? by William M. Greathouse
- Spiritual Geometry: Evaluating the Wesleyan Quadrilateral by Vic Reasoner
- Wesley and the Quadrilateral: Renewing the Conversation (1997, ISBN 0687060559) by Campbell, Jones, Maddox, & Miles
- The Wesleyan Quadrilateral Revisited, by Jeff Gauss