Why must I specify workbook here?

I'm stepping through very basic VBA to re-learn some essentials.

I have two workbooks open, each with a couple of worksheets. No duplicate names. The two open sheets are called Track Data and TEMP respectively.

Why does this fail with subscript out of range please?

Worksheets("Track Data").Activate

It works if I specify the workbook.

Workbooks("TEST track sheet copying.xlsm").Worksheets("Track Data").Activate

That involves finding its name and carefully pasting it into my code, so I'd prefer not to do so. Why must I specify it in this situation please? In what circumstances can it be left out?

Terry, East Grinstead, UK