Wilhelm Wundt's conception of affect

Chapter 4 endnote 36, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

In the science of emotion, the term “affect” is sometimes used to mean anything emotional. In this book, we limit the term to a specific meaning: a change in your internal environment that you experience as feelings of valence and arousal. This modern conception of affect was developed by Wilhelm Wundt.

The modern conception of affect developed in the 19th century with Wilhelm Wundt, who made seminal contributions to the emerging science of psychology and conducted the first formal psychology experiment in history. He began his career as a physiologist and initially believed, like William James, that emotions were perceptions of raw sensations from the body. But Wundt quickly moved away from the physical to the mental counterpart of these sensations, which he called “affect” after the German word Gefühl, meaning “feeling.” Wundt described affect as “simple feelings” of pleasantness or unpleasantness (called hedonic valence or just valence), arousing or subduing (called arousal), and strain vs. relaxation (which corresponds to intensity). Note that arousal and intensity are distinct properties, as one can be intensely calm (low arousal) or intensely excited (high arousal).

Wundt considered affect to be a sixth sense along with sight, hearing, smell, taste, and touch, and wrote that all these sensations are “different points of view from which we start in the consideration and scientific treatment of a unitary experience.” Wundt was basically saying—though not particularly clearly—that every moment of experience unifies all your senses, and affect is a part of this integrated whole. He made these claims over a century before anyone had discovered the multimodal integration network or neural basis of multisensory processing. Kant (also not known for his clarity) called affectively infused perception the “transcendental unity of apperception.” According to Wundt, these independent qualities were not “ingredients” of affect, but features of a simple, unified feeling that changes from moment to moment as part of consciousness. This way of describing affect, as having some valence and arousal, has been one of the most powerful ideas in psychology over the past century. It is so commonly assumed that people often fail to provide scientific citations anymore.

Over the years, scientists have sometimes misunderstood valence and arousal to be causes or consequences of affect (they are not); to be synonymous with emotion (they are not), or even a sufficient description of emotion (again, they are not). For Wundt, valence, arousal, and intensity were features or qualities of the simple feelings that arise from internal bodily sensations. People are, wrote Wundt, “never in a state entirely free from feeling.”

Scientists like Wundt and some other constructionists considered affect an ingredient of emotion. They believed that emotion was constructed when affect made meaningful in some way. This is similar to the psychologist Jim Russell's theory of emotion. Originally, I had the same mindset before studying neuroscience and refocusing on interoception as a basic ingredient of emotion. Affect is not sufficient for emotion, and is certainly not unique to emotion, but it is necessarily a property of emotion. Think of affect as a low-dimensional representation of interoception that includes two features, valence and arousal. Affect gives us a little bit of information about what it feels like to be angry or fearful. It does not tell us how we experience anger and fear as different from each other, nor how one instance of fear differs from another.