I must be missing something here ...
Sorry for the length of this post, but I need to explain what I am seeing to ask why.
I downloaded samples of some of the books posters recommended and am glad that I did that because reading about stuff that I thought I knew about gave me some information I did not know, at least about what options are available when using Photoshop. I learned perhaps 6 or 7 new thngs just reading the samples of the books. But one of those things that I learned has me puzzled. Not about what is happening, but about why it is happening and something similar is not.
One of the exercises involved loading a single image and adding a new empty layer to it. I know that if the empty layer is slected I can not use most filters and adjustments because there is there is nothing in the layer, but the example told me to check the Sample All Layers checkbox at the top of the image. That was something new for me as I had never noticed it before. It then had me use the Spot Healing Brush to show me that even with a new layer selected I could use some Tools to modify an image. That was also something new for me. In fact I was so surprised that I decided to try some other tools, using the same configuration.
I tried ACR, but it did not work. I tried both the focus and blur tools and they did not work. None of that surprised me too much because the new layer was empty and it seemed as though I could use the Spot Healing Brush because did not change the image layer but allowed me to paint on the new layer (which is what it did. Hiding the image layer showed me the Spot Healing Brush changes on the new layer). What surprised me, and what I don't understand is why the Spot Healing Brush worked but the Healing Tool did not. They both seem to do the same sort of thing - sample the base imae and paint a change - but since they are so similar why does the Spot Healing Brush work and the Healing Tool not work? If the Spot Healing Brush and the Clone Tool both work on the new layer, why not the Healing Tool?
The reason this bothers me enough to post the question is that I feel that I am missing some basic concept about what is happening, and that not knowing may also mean I don't understand other similar things.