Reason has a multifaceted definition. In our use here, we shall have it to mean "A process of applying logical thought". Where we use "Logical" we refer to only that which is consistent, by consequence that it has no internal contradiction.
If we examine this by further consequence we immediately realize that its common uses, even that which is defined thence, requires that nothing be done to determine its "external" consistency. Such that any given line of reasoning can be logical, and yet still not be consistent with external observations of the world.
Therefore, a third process is required to achieve consistency with the external world. For any given thought can be made to be consistent with other thoughts, and yet have no ties to observations we might otherwise make. In this, we have empiricism, or the philosophical process of attempting objective observations of reality.
All three processes share in one specific quality in order to be valid: Consistency. A given "set" of reasoning is only valid if consistent with its fellows, while a given "set" of observations are only valid if consistent with its fellows.
If the above is taken holistically, as a mental "unit" of thought, we can apply its consequences to make some interesting observations. Namely on my mind, that in which scientists have been disparaging philosophy. For both require the same quality, and one invariably leads to its fellow.
I further would conclude that philosophy, if defined by reason, must in turn endeavor to define itself by consistency to be valid. To be of use to the real world, must further use those consistent conclusions to make observations of it. Therefore, any philosopher can be a scientist, and any scientist must become a philosopher.
For as a scientist observes, he must provide a reasoning which is consistent with the observations. In endeavoring to do so, and to ensure reliable success instead of accidental, he must do the same which a philosopher must do with the intangible.