Wordsworthian poetry is very sentiment-oriented (it was almost certainly this aspect of it that helped J. S. Mill out of his nervous depression). But it is not the less philosophical in aim for being sentimental in means. As Wordsworth puts it in the Preface to the Lyrical Ballads:
Aristotle, I have been told, has said, that Poetry is the most philosophic of all writing: it is so: its object is truth, not individual and local, but general, and operative; not standing upon external testimony, but carried alive into the heart by passion; truth which is its own testimony, which gives competence and confidence to the tribunal to which it appeals, and receives them from the same tribunal. Poetry is the image of man and nature.
This poetic search for the self-authenticating face of truth is where sentiment comes into play: the Wordsworthian poet attempts to portray things with, as he says elsewhere in the Preface, a "colouring of the imagination," so that even ordinary things become striking, thereby, as he says here, to carry the truth "alive into the heart by passion." The poet does this by respecting the principles of association at the basis of the imagination, particularly insofar as they govern the mind in a state of genuine excitement, one that follows from our perception of the like in the unlike and the similar in the different. Poetry attempts to capture something of this excitement of the strangely familiar and familiarly strange world, recollecting the feel of it in a quiet moment ("emotion recollected in tranquillity" until it becomes vivid and living again). This pursuit of what pleases is itself supposed to be philosophical:
It is an acknowledgement of the beauty of the universe, an acknowledgement the more sincere, because not formal, but indirect; it is a task light and easy to him who looks at the world in the spirit of love: further, it is a homage paid to the native and naked dignity of man, to the grand elementary principle of pleasure, by which he knows, and feels, and lives, and moves....We have no knowledge, that is, no general principles drawn from the contemplation of particular facts, but what has been built up by pleasure, and exists in us by pleasure alone.
One of the primary tasks of the poet, if this is true, is to capture things we already know about the world, but make us realize them anew, as the exciting and interesting things they are. The poet drives our interest in the world as fitting to the human mind, and in the human mind as fitting to the world: he therefore captures the very essence of inquiry, and makes poetry "he breath and finer spirit of all knowledge" and "the impassioned expression which is in the countenance of all Science". The poet takes science and knowledge and makes them genuinely human things; a necessary function, without which most people will necessarily be shut off from them.
All of this does have the implication that poetry deals with appearances, not realities; as he says in the supplementary Essay of 1815, it concerns itself with things "not as they exist in themselves, but as they seem to exist to the senses, and to the passions". This does have a potential danger: poetry is spontaneous overflow of emotion, yes, but it is spontaneous overflow within the context of disciplined understanding and reason. The common failure to recognize this makes it difficult to recognize true poetic genius, the kind that makes poetry a companion and indeed mother of knowledge:
Genius is the introduction of a new element into the intellectual universe: or, if that be not allowed, it is the application of powers to objects on which they had not before been exercised, or the employment of them in such a manner as to produce effects hitherto unknown. What is all this but an advance, or a conquest, made by the soul of the poet? Is it to be supposed that the reader can make progress of this kind, like an Indian prince or general—stretched on his palanquin, and borne by his slaves? No; he is invigorated and inspirited by his leader, in order that he may exert himself; for he cannot proceed in quiescence, he cannot be carried like a dead weight. Therefore to create taste is to call forth and bestow power, of which knowledge is the effect; and there lies the true difficulty.
Poetic genius -- and Wordsworth clearly counts himself (and not incorrectly, it must be said) -- unites past and future: all our sentiments and passions, which have been in us for all of human history, clothe even the most up-to-date knowledge in striking form, intimating new approaches. It exhibits "that accord of sublimated humanity which is at once a history of the remote past and a prophetic enunciation of the remotest future" and therefore is rarely appreciated in the present.
Wordsworthian poetics, then, is not just verse but a method of philosophical description, whereby one attempts to express the features of the world in the way most suitable and appropriate to the human mind, and to express the features of the human mind in the way most appropriate to their interaction with the world; it manages this by basing itself on actual experience of the world. In experience the world we sense and feel; and the feelings and emotions with which we experience the world can be re-captured by a means of description that takes into account the minds own processes of association and uses them to put the mundane in a striking light. We re-experience, but in a sense we do so by again experiencing it for the first time. This paradox is rooted in the fact that the poet uses similarities and dissimilarities, likenesses and unlikenesses, to describe the world we have already experienced; this brings it again to mind and in a way that leads us to recognize again things we have perhaps forgotten or come to take for granted.
We tend, of course, to think of philosophy as concerned with good arguments; but there is an excellent case for saying that good description is also essential to it. If nothing else, otherwise good arguments regularly run aground by being based on poor and misleading descriptions of the world. And description, too, can shade into argument. Wordsworth's Intimations of Immortality from Recollections of Early Childhood is not often recognized as an argument for the immortality of the soul, but it is. Wordsworth somewhere explicitly talks about the elevations of the sentiments and the affections toward eternity that are produced by faith as a "presumptive evidence of a future state of existence" and the ode is itself an attempt to describe those features of childhood experience that intimate our suitability for life beyond this mortal frame of nature, those features of our lives that give meaning to the idea that we seem to be fit for something more. It is a philosophical argument. It is, however, a philosophical argument built on extraordinarily subtle aspects of our experience; capturing these properly in description is beyond the ability of most people we think of as philosophers. It is a purely philosophical argument that only a master poet could write.
Philosophers, and certainly philosophers today, are not taught to pay attention to intimations, so proper presentation of an argument that is based on intimations is not something most philosophers can do. Trying to remedy this was one of the primary goals of the Romantic philosophers, who argued that poetry, science, and philosophy must come together. It is this that makes Wordsworth a Romantic poet. And it is this that makes Wordsworth a Romantic philosopher.
How exquisitely the individual Mind
(And the progressive powers perhaps no less
Of the whole species) to the external World
Is fitted:--and how exquisitely too--
Theme this but little heard of among men--
The external World is fitted to the Mind;
And the creation (by no lower name
Can it be called) which they with blended might
Accomplish:--this is our high argument.
Knowledge for us is difficult to gain--
Is difficult to gain, and hard to keep--
As virtue's self; like virtue is beset
With snares; tried, tempted, subject to decay.
Love, admiration, fear, desire, and hate,
Blind were we without these: through these alone
Are capable to notice or discern
Or record: we judge, but cannot be
Indifferent judges.
