That's essentially what I meant. By order I mean they...okay let's see if I can articulate this properly. The three components of the soul: heart, mind, and spirit, symbolize the Id, Ego, and Superego.
If one of these creatures retains their mind then they remember their human life. This gives them an entire life's worth of experiences to draw upon and, in the case of the light beings, a purpose.
Beings of light and order are essentially a race of superegos. They lack emotions and tend towards long term planning or achieving a common goal. Their goal varies between individual or groups though those without minds usually fall under the sway off an Angel(one that does have a mind). This Angel is essentially a mentor and gives them a purpose, achieving the Angel's goal. The Angel's goal generally branches off from what they did in life or something they wanted to do in life. For instance, if they were violently murdered by a person they might seek to return to Earth and end that person's life.
Dark beings are Ids. The ones without minds are just violent monsters that attack each other and are ruled by emotions. Ones with minds generally exhibit a higher level of sentience because they remember things from their past life. They know what kinds of people they were and have more of an identity. Regardless of their state of mind they tend towards short term goals and instant gratification.
So to answer your question, they both have the capacity to think rationally, mind or no mind. It's just the ones that retain their human mind have all their memories and thus an identity, making it akin to being born a baby that has to figure things out again or being born a baby with all the experience and knowledge of an adult. They also tend towards certain kinds of thinking depending on whether they're the light or dark half.
@Ettina and melloncollie: Well they will die eventually no matter what. I said if they stay they're gradually purified and when they're fully purified their body disintegrates and the fragment goes to the essence. If they leave and go to Earth they can potentially live forever. However the Demons, as said before, are Ids. They can think rationally but they tend towards violence so it's usually just a matter of time before something happens.
As for fearing the return they react slightly differently to it. Light beings usually see it as something that will happen eventually so they won't fight it. However if they want to continue their goal they might try and put off the return.
Dark beings like to be happy and generally like existing and doing fun things. For example, even among the most devoutly religious people in this world, many will say they fear death even though they realize it isn't the end. It's for this reason that they go to Earth. Sure they might get killed by a Reaper but it's something that might be prevented if they're strong enough.
Sorry for the long post. I know exactly what I mean but I'm having a hard time explaining these ideas to others.
edited 30th May '11 9:45:11 AM by Kostya