Typographer John Langdon designed this ambigram for the Department of English & Philosophy at his institution, Drexel University.
“This illusion was a particularly difficult challenge,” he told Brad Honeycutt for The Art of Deception (2014). “My attempts to create more ‘conventional’ (rotational, mirror-image, etc.) ambigrams for these two words were unsuccessful. But my personal investments in both philosophy and language seem to inspire me to some of my best work. This ‘perception shift’ ambigram was very difficult to develop, but my stubborn persistence finally paid off. The two words ‘philosophy’ and ‘English’ can be difficult to discern, but with a little patience and a voluntary perception shift, finding them is particularly satisfying.”
There’s much more at Langdon’s site.